Rattling Noise coming from under cab

Hello everyone, I am having an issue with my truck, and am hoping to glean some insight from fellow owners. First off, my pickup is a 1991 K1500 Silverado with the 4.3L engine and 5-speed manual transmission. I took it to work a while back, and on the way home I started hearing a rattling noise under the cab that didn't sound good. You don't hear it when taking off from a stop, or really at all in first or second gear. It begins rattling under throttle in third gear, and starts to get worse when you go up to fourth, at which point it seems to start to lose power a bit.

So, here is what my thought process has been thus far and what I have done to try to correct the issue:

-Thought 1 > Maybe a loose heat shield or something broke loose in the catalytic converter, causing excessive back pressure, explaining the power loss under heavier throttle. It was the original stock cat, and the exhaust piping had been replaced in segments all over the place including the y-pipe on one side.

Solution: Exhaust was a cobbled mess anyway and needed replacing rather than patching further, so I replaced the manifolds, y-pipe, catalytic converter and length of pipe back to the muffler (which was still good and had only been replaced a couple years ago) with brand new parts. Not upset that this didn't stop the rattle because it needed done anyway and I'd been putting it off.

-Thought 2 > Maybe some fluids were low or worn out, and the rattle was from inside the transmission or transfer case.

Solution: Went ahead and drained the transmission, transfer case and front differential while I was under there and refilled with new fluid. (Again, probably overdue, so not too upset that it didn't correct anything, as that was probably way past needing to be performed. Rear diff fluid had been replaced a year ago)

-Thought 3 / Solution > Possibly problem at the engine (as sound is coming from front center of the cab) Last oil change was only 1200 miles ago, but the truck sat for a long while when I had the bed off the frame and was performing surface rust repairs, so I figured I'd just do it myself again for good measure. Also thought one of the spark plugs might be going bad and causing issues, so I replaced all plugs and plug wires too because I couldn't remember when I'd done that last (probably 2009 or so).

Truck now drives and shifts smoother than it has in a LONG time, yet the rattling sound persists under the conditions described at the beginning. I also checked the drive shaft U-joints for any play in them, but they are all solid and tight. I'm starting to run out of ideas on what this could be, but not upset about any of the maintenance that I've performed trying to reach a conclusive solution, as it was all stuff this old truck needed done to it, and it made me get off my rear and do it on my days off.

I am open to further suggestions/speculation on what could be the cause of this issue. It is definitely starting to become a real head scratcher, and besides the few runs around the block to see if any of the aforementioned ideas helped anything, I haven't been driving it anywhere until I get it figured out. (I have another much newer truck that I've been driving to work in the meantime), but I still want to figure this out soon so I can drive my Chevy again. It may be the oldest vehicle I own, but it was also my first, and my favorite, hence why I've been putting so much effort into it. Any help figuring this out would be greatly appreciated.

Is it possible that the rattle is from something as simple as the handbrake cable, or fuel filter and bracket moving around under vibration? Can't remember what else is under there that could come loose and move. Maybe steering column and associated wiring are hitting the toe board? Good luck on figuring this out.

Still have the jack & stuff behind the seat? Is it locked down tight?

Jack is secured, the sound is coming from under the cab, towards the center front somewhere. I am going to put the rear end up on jack stands and have a friend listen from the outside to see if we can figure it out.

I would check the drive shaft for binding. Mine is kinda out of wak and when i have some weight in the back it rattles really bad when i decelerate

Okay, I have an update, and got my father on board to help me try to solve this. He was the original owner of the truck, so he knows all the details about it too. We put the rear end up on jack stands, and ran the truck in gear, in place, and tried each gear at 2k rpms. We could not hear the rattle at all. It sounded just smooth and running well (probably due to all the tune up and replacement maintenance I've recently done elsewhere and mentioned in the first post).

We took it out for a spin, and the loud rattling is still present, but only when giving it throttle to speed up. When coasting, or with the clutch pressed in and given a little throttle, nothing. Slight rattle when maintaining speed, and major rattle when increasing speed. So basically only when torque is increasing does the rattle appear, and gets louder the more the throttle is given.

Still confused as heck what could be causing this, but hopefully this helps narrow things down a bit.

An unrelated issue, a brake line segment that didn't get replaced last time I replaced all the brake lines broke and I lost brake pressure when I came back from the test drive. So...going to go ahead and fix that today.

I welcome any advice or suggestions based on this updated information. I will check back shortly after I get the brake line fixed. Thank you to anyone who can help.

Well, the problem is being solved, finally. I gave up trying to figure it out myself, and took it to a local reputable garage that has done good work for me in the past to have them look at it. As it turns out, it WAS the engine. Despite sounding good when I had it up on jack stands, it was making noise when actually moving the truck on the road because it was knocking bad. The PSI was super low on a couple of the cylinders, and it was totally unbalanced. They said it would need a total rebuild, or a replacement. I wasn't totally shocked by this, because it wasn't the original engine anyway.

A bit of history, the original engine, blew a rotted gasket while I was on the freeway at cruising speed many years ago. I knew right away something was wrong, but was in traffic where they had the concrete shoulders up with nowhere to pull over. I got to the next exit and pulled off immediately, but it was too late, and the damage was done, all in the course of two miles. While that was a gut punch to someone who meticulously keeps up with oil changes and regular maintenance, it was just bad luck, and a wrong place/wrong time type of situation.

I replaced it with a used engine from a wrecked junkyard truck. I got 4-5 years use out of that one, and it only cost me a few hundred dollars, so I'm not surprised that it was the problem. I have no idea the history of that engine before I took it and used it. The garage said they could get another used 4.3L for me, but I didn't want to set myself up to have a similar problem in another few years going the same route again. Plus, having someone else do it was going to cost an arm and a leg. I told them if I was going to dole out that much money, I'd rather pay a little more and just do a crate engine replacement. Problem there is that no more crate 4.3's around now they told me, and used or used rebuilt is all there is.

Soooo, bottom line, I'm having a crate 350 small block put in and the conversion work done by them. At least with a new engine, I can maintain it properly from the start and hopefully have many, many more years of happy driving with my truck. It should be done later this week, and I can get back to finishing the body work touch ups and a repaint myself before too long.

If you got to pay either way, may as well be an upgrade, right? I'm anxious to get it back and start driving it on a regular basis again.

Never noticed this thread before, but found it very detailed and interesting. The methodical approach taken by LoneCynic should help us all when we have to diagnose issues with our trucks.

Too bad it was the engine, but it sounds like you're ok with the replacement, and will have a great truck to enjoy.

They called and said it was done on Friday, but I was attending an event on the other side of town and couldn't get back before they closed. They aren't open on the weekend, so it has been a long couple of days waiting. I am planning to go pick it up after work tomorrow. I'll let you all know how it turned out.