Transmission Knocking Noise? (Getrag 5LM60)

[DerekTheGreat]

Hey everyone, long time no chat!

We begrudgingly sold/traded our C3500 for some cash and an old Ferd F250... It wasn't long until we got our next project though, a 1989 Chevy K1500. I'll see about getting some pictures uploaded soon. Anyway here's the issue:

When the trans fluid (Stickshift, believe it to be the oddball Getrag 5LM60 with the 4.00 first gear.) gets hot and with the engine idling in neutral you can hear it knocking, sounds either like a "clack clack clack" or pretty guttural, like rod knock. I actually thought it was rod knock up until one day I put my foot on the clutch and the noise went away. What is it? We've put over 2,500 miles on it so far with no issues and it goes away over 1,100 rip'ems or so and we don't really hear it when it's in gear. However, it makes a pretty angry noise if you try to throw throttle at it in gear below 1,100 rip'ems, like an angry rattle or something, very similar to what spark knock sounds like but angrier. Truck has 224,000 miles right now so it's no spring chicken. I've had all the fluids changed and am running what it calls for, GM Synchromesh. However, when they first changed the fluid the guy put gear lube in it and it shifted like trash when cold, like our old SM465 did. Once that was drained out and Synchromesh put in it's place it shifted great again but that's when I began to notice the clackity clack. I was thinking of putting a 50/50 mix of that and gear lube in, to help quell the noise, what do you guys think?

Also- Could be related or not but the engine has a really rough idle, almost like a miss yet when you pull the wires one by one I've found there's no dead cylinder, as the idle gets even worse.. Not sure what it is as the engine runs great at slightly higher rpms.

Derek

 

[BerHntr]

I don't know what the sound is but I'm pretty sure mines got the same problem. I suspect it to be partially due to the throwout bearing but i also think there may be some bad bearings in mine as it is quiet as a mouse in 4th gear only. I think that's the only gear that doesn't involve the counter shaft, i may be misinformed though. My solution will be to put an nv4500 in it's place when I do my ls swap but I am curious to know what it is.

 

[RichLo]

Syncromesh is good stuff on newer transmissions but the old ones have pretty loose tolerances. I would try either your 50/50 idea or 50/50 syncromesh and Lucas oil stabilizer.

I've also heard about a banana peel trick that old hot-rodders swear by.

 

[DerekTheGreat]

Banana peel? Like E-lect-ri-cal banana? (They call me Mellow-Yellow...)

I got my lubrication info straight from the horses mouth, the GM Service manual... From the trolling I've done it seems that anything other than Syncromesh won't get to the bearings and what not and will cause crappy shifting like I noticed with straight gear lube. Could just be that my syncros are worn out for 1, 2 & 3 as 4th and 5th went in just fine and everything was easy once the gear lube warmed up. But during the warm up period I was certainly concerned my trans wasn't getting lube. I'm pretty sure I got the angry spark-knock like noise even with gear lube but not the knocking. There's also what seems to be gear whine in 1st and second, which didn't seem to be there with gear lube. My limited research points to these things barfing the input shaft bearing and perhaps that's what my trans is doing, so I'm hearing the knock because it's slapping around & the rough idle is only helping that. Yet that doesn't entirely make sense because it's not like the crank is loose and why would the input shaft be able to slap around? Perhaps its the throwout bearing like BerHntr mentioned? I thought those only made noise when the clutch was being pressed?