Chevy 350 light knock

[TrucksNameIsEarl]

Got an 85 GMC K1500, 305 from the factory, swapped rebuilt 350 in it currently, it's developed a light knock! On start-up it's not there, after about 10 minutes of driving it's really noticeable (audible and feel it under your feet in the driver's seat) when braking. Now it could be that it's always there and the noise of the engine and exhaust drown it out, but you can only feel it when braking. Any ideas? Hoping it's some kind of detonation issue. Going to be changing out the spark plugs this weekend, throw some oil thickening agent in the block, and try a slightly higher octane gas.

Any other suggestions would be much appreciated!

 

[TrucksNameIsEarl]

Forgot to mention.. I've taken it to a mechanic buddy of mine to make sure drive components and suspension are good to rule out any possible drive-shaft or random suspension tolerance issue, but he seems to think that all checks out

 

[Bextreme04]

There is nothing I can think of off the top of my head that would make an engine "knock" so hard that you can feel it in your feet only on deceleration. The engine is under the highest load at acceleration and should be basically free wheeling during deceleration. You might want to check the torque converter bolts, transmission bellhousing bolts, and engine/trans mounts

 

[Ricko1966]

Look for a cracked flex plate.

 

[TrucksNameIsEarl]

Thanks for the replies, guys!

After driving around a bit and seeing if I could recreate it, it occurs when braking hard. Braking softly doesn't seem to upset the vehicle. I will check out mounting bolts and flex plate!

 

[TrucksNameIsEarl]

Ha! Ok, found a bolt missing on the transmission mount! 12mm bolt I presume? I'll pick a bolt up tomorrow and give updates

 

[1987 GMC Jimmy]

Ha! Ok, found a bolt missing on the transmission mount! 12mm bolt I presume? I'll pick a bolt up tomorrow and give updates

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