Possible clutch or transmission knock

I have a 2nd gen 4cyl and once the engine gets good and hot and the a.c. is running, while sitting in neutral with the clutch engaged it makes a sort of knocking noise. It doesn't follow engine rpm as when I press the gas the noise stops. When the clutch pedal is drepressed the noise stops. It only makes the noise when sitting still in neutral with the a.c. on. Its not the a.c. compressor. Any ideas?

 

Sounds like a throwout bearing.

 

Is it more of a ticking sound or knocking like you say? I have a 'tick tick tick tick tick' sound coming from my transmission that speeds up with the RPMs but goes away with throttle applied or the second I touch my clutch pedal. I'm leaning toward throwout bearing as well, or possible a cracked flywheel. Either way, from what I gather it's a cheap part but expensive to fix (have to drop the trans) and it's best to hold on out repairing it until it's time for a new clutch, if possible. Aside from the annoying ticking sound, I haven't had any issues related to it for the last few months that it's been around.

 

The clutch was replaced by the dealer I bought it from about 1 year ago. I'm new to owning a manual and a toyota at that. I've worked on cars but never the transmission or clutch before so I'm not sure how to fix or be sure if a throw out bearing is the problem.

Eon_blue, it is a knock for sure. Not a suttle sound either. You can hear it clearly over the radio and other cars hear it as well. But the moment you disengage the clutch the sound stops.

 

How many miles on the vehicle?

 

Throwout bearing for sure

 

If it only makes noise with A/C running, it might be the alternator. Mine made noises with A/C on because of the extra load, but it wasn't a knock. It was a high pitch whine (not TOB related). Just something to think about.

 

108k on truck and about 15k on new clutch

 

I say T/O bearing too.

 

For curiosity note what idle rpm it runs when the ac is running. Turn off ac leave it in neutral clutch pedal out rev engine to same rpm example 900rpm see if noise matches the original sound? I know with all my manual transmission Toyotas they have all had noisy input shafts in the transmission when hot under the conditions you describe it's most noticeable to the ears. I've had 7 manual transmission Toyotas for comparison.

 

A lot if not all of the 5 speeds do this with the a/c on. Push the shifter towards a gear like you're going into it but not all the way and it will stop, or push in the clutch or go a/c off. It seems like a shaft in the trans vibrates at a/c idle in neutral.

 

Good amount of miles. Like others said, sounds like it could be T/O Bearing. Also, if your clutch was replaced, make sure the pedal was adjusted correctly, particularly the free play. If not, it could have been putting extra wear on the T/O bearing during the 15K since it was replaced.