[aceaceca]

I do not see an engine tech forum so I decided to post this here. It is quite obvious if you have been on this forum for some time that there are some members here who have significant knowledge in the engine department. Not only is their knowledge base amazing but it spans years and years of dealing with automobiles. For many, any problems that arise are just another been there, done that ,fixed that, situation and they move on to the next problem. I am not in that category by any means but I have worked on enough engines to get by. Well, I had a recent gotcha moment with my 65 GTO that has been family owned since new and serviced solely by me it's whole life. I am going to relate the problem I encountered with the engine and I am curious if one of you veteran car guys knows right of the bat what I was dealing with. I know the solution to the problem as I fixed it .

Problem: When the motor drops down to a slow idle it knocks as if a bearing were spun or a wrist pin loosened. Not a bamb bamb knock but definite audible knock. When the idle comes up a bit the knocking stops. When you hear it you think oh no, the motor is toast. What do you think was causing this?

I will give you a critical hint so you do not go off on a tangent. The cause is not in the long block.

[dataway]

Loose flywheel/clutch/flexplate

[Tom Vaught]

I had a drive shaft U-Joint do that one time. But this happened only at low speeds and it sounds like your noise is a idle noise with the trans in neutral. If a Manual Trans vehicle is the noise with the trans in neutral? My vote would be the front input shaft bearing has a problem.
Tom V.

[equidr]

Loose balancer pully.

[AG]

Check for the exhaust leak that sounds like a rod knock.

[chrisp]

Plug wire loose will cause a knocking sounds

[71 Goatman]

X2 on the loose balanced pulley

[Gary H]

Harmonic balancer ring separated and rattling

["QUICK-SILVER"]

Chunk gone out of a fan belt?

Clay

[Sirrotica]

I worked at A Pontiac dealership in 1970 while I was on Co-op in my automechanics class as a senior. It was quite common for a car to come in that has loose torque convertor bolts during this time with the exact same noise you're describing.

Later on in my vocation I've run into a few Pontiacs and chevys with cracked flex plates that also sounded much like a rod knock, but not exactly the same.

In the early eighties the Iron Dukes were pretty famous for a carbon knock because of carbon buildup on top of the pistons. I realize you said it wasn't in the long block, just thought I'd throw this in, as I've run into it a few times also.

[STEELCITYFIREBIRD]

Any idea what area it is coming from?

[aceaceca]

Good guesses so far. But no cigar yet. I have had the loose balancer ring and the big time spider cracked flex plate on this car though.One more day and then the answer. Another hint might give it away. Although, it was something one would never expect to be the cause. If I had not seen it and fixed it I would not have believed it. So that is a good clue.

[aceaceca]

The knock is in the engine at idle.

[PunchT37]

Broken spring on heat riser valve. Do I win?

[Schurkey]

1. Engine-mounted fuel pump knock

2. Failed bearing(s) in alternator or A/C compressor

Neither typically knocks only at idle, but it's possible.

The guy I bought a Vortec 5.7L short-block from pulled the engine for a rod knock, and then found the flexplate was cracked around the crankshaft bolts.

[421mike]

Broken motor mount? Do I win?

[dataway]

Exhaust pipe knocking on the transmission crossover.

Exhaust heat riser valve flapping around (broken spring)

Broken spark plug.

[jonmachota78]

Guy knocking on the window telling you the lights green?

[torqhead]

Originally Posted by jonmachota78

Guy knocking on the window telling you the lights green?

^^^Hilarious!!!

[Gary H]

Intake exhaust crossover gasket leak