Hello. My truck is 1996 4.9L 5 speed od manual, xl, and about 100K miles. Recently started hearing single "thunk" just as the engine fires up. The noise seems to come from around under the shift lever. The noise doesn't happen every time, and it doesn't matter if it's cold or hot engine.
Reading the web suggests:
1. loose starter bolts - 1-1/2 yrs new autozone starter
2. loose motor mounts - original and no problem/noise when driving
3. old u-joints - no problem/noise when driving (although there is a thunk when turning left) and when there is no "starting thunk". Rear replaced 10 yrs ago.
4. predetonation
5. damaged starter gear?
6. starter solenoid piston?

The transmission had the clutch pressure plate changed when slave cylinder had to be replaced about 3 years ago.
Only thing I know how to check would be loose bolt and may be u joint. I'd like some advice before trying a new starter. I'm hoping the transmission doesn't have to be checked. So far, best guess this newbie can come up with is that starter solenoid spring is no good, therefore, the thunk as the piston retracts.
Thank you in advance.

 

sounds like the ring gear around the flywheel has a cracked weld or jammed tooth or two.

could be starter piston/solenoid...or trans main shaft bearing or pilot bushing/bearing but you should feel or hear that most of the time.

I would crawl under and remove the clutch inspection plate/dust cover and check the ring gear, as that would be cheap to rule out. It should be "weldable" while on the truck as long as it is still attached at most of the welds.

 

Thank you for the reply. I'll check on it.

 

Still haven't looked at it. Just trying to get the operation in my head before the visuals. Am I guessing correctly?:
The starter turns the flywheel which turns the engine. Electrics and fuels come in and the engine fires up. The clutch is still in neutral, so the noise can not be the clutch or the transmission.
The thunk comes only when the engine is warm or hot. There is no noise (I remembered wrong) on a cold engine. 3 cold engine starts and no thunk. Also, the noise comes just after the engine fires up. I assume at this point the starter bendix gear is disengaging from the flywheel.
So my guess is that the "thunk" is:
1. bendix gear and ring gear are not separating smoothly - problem with the starter?
2. bendix gear shaft is retracting into a faulty spring? - replace starter?
The engine heat can change the shape or size of the parts, so is there other moving parts or bearings that move with the flywheel that might make this single noise?
Again, thank you in advance.