‘99 on the road finally, now rattles in first

I’ve finally had time to get my 2500 back together and on the road, and I’m hearing a weird rattle noise I need help diagnosing. I don’t think the truck rattled when I brought it home a few months ago, but it ran so poorly on the drive home I may not have noticed.

It only happens in reverse and 1st under moderate load. If I baby it or floor it, there’s no rattle. I can also lug it in all the other gears and hear no rattle. I initially thought it was something in the front drive (fan clutch?) or the exhaust vibrating against something, but haven’t found any culprit. My buddy thinks the sound is coming from the TC dust cover, but I double-checked tightness on those bolts. I’m not feeling anything, if that makes sense…just hearing the sound. Any suggestions?

Double check the exhaust, I had a muffler clamp that was loose, drove me nuts for a few months until I crawled under and put a wrench on every nut.
When I swapped tranny's and started it up the first time, it was definitely rubbing, due to catastrophic failure must have gotten banged up. Easy enough to pull it off to eliminate it as a culprit.

Unfortunately I’m back to this…looks like the flex plate is cracked. So I guess I’ll be dropping the trans at some point.

Aholter90 said:

Unfortunately I’m back to this…looks like the flex plate is cracked. So I guess I’ll be dropping the trans at some point.

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Before you do all that work. Pull the starter and check the bendix. Had a friend pull the trans in his 96 Yukon for what he thought was a broken/cracked flexplate. The starter bendix had failed and it was bouncing off the flexplate.

L31MaxExpress said:

Before you do all that work. Pull the starter and check the bendix. Had a friend pull the trans in his 96 Yukon for what he thought was a broken/cracked flexplate. The starter bendix had failed and it was bouncing off the flexplate.

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Did it look like this one?

Piratehunter said:

Did it look like this one?

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That looks like kickback from a bad crank sensor. GM TSB on the Vortec era trucks for that.

His just had a bad or weak return spring and the bendix was sliding back and forth freely.

@L31MaxExpress good to know. I got a P1345 code with that explosion. Since then I had the top end apart to replace lifters, had the wiring harness disconnected from the crank sensor back to the coil to access everything. poked the dizzy back in and reconnected all up, starts, but still P1345 code. Need to set the cam retard offset. will double check connection on the crank sensor. Had installed a new one with the rebuild about 700 miles ago. any other tips on this? sorry to hijack

L31MaxExpress said:

That looks like kickback from a bad crank sensor. GM TSB on the Vortec era trucks for that.

His just had a bad or weak return spring and the bendix was sliding back and forth freely.

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Interesting, I never heard about the crankshaft position sensor TSB. My Burb had a bad spot or two on its flexplate, was hell to start when it hit them. I figured it was double firing from bad cap and rotor as I had needed to replace those early on. Long story short, its starter ended up looking like that. I had a friend who had access to a lift, replace the flexplate, and haven't had any more problems with that.