Hello,

So I am currently considering purchasing a 1990 Vette convertible base model non ZR1 with the 6 speed it has about 43000 miles on it. I am going to be purchasing it from a family member and he is letting me drive it a bit to see how I like it and if there are any issues with it. He had purchased the car and never really drove it.

After driving the car a little bit and it warms up, then if I stop and let it sit I hear a "knocking" noise almost like someone hitting something metal with a hammer. It can be heard while sitting in the driver seat. There does not seem to be a pattern to it and it sounds as though it is just a random sound. When the car is cold and I start it I can not hear the sound. Also when I shut the car off it sounds almost like a rattle sound. It seems as though it is the loudest on the passenger side and if I am under the car.

I had opened the hood a couple days ago and realized that the plug wires were bad (arching from wires to block and other metal bits under the hood). Changed the wires and no change to the sound.

I have attached video links to both of the sounds. The darker video is me laying on the passenger side on the ground with the phone in the area of the cat. Any help is greatly appreciated as I would love to buy this car, but if it is a major issue I would rather pass and try to find another one. Thank you, Brian

 

I hear all kinds of crap.
Something sounds like a clunking noise in the first video that reminds me of a broken catalytic converter. Like the catalyst material inside the converter is broken and banging around. This makes sense if its on the same side as the failed plug wire. Its impossible to be sure though.

 

Yeah of course the fan was kicking on while attempting this as well as well as other engines noises. It is the loud "bang or knock" that is most concerning like metal on metal or rocks in a coffee can I guess may describe it. If it is only a cat I am not overly concerned but if its internal in the motor or trans I will probably move along.

Should have added initially that there are no check engine lights and that when the car is moving it is absolutely fine and runs strong.

 

Rocks in a coffee can is a good description. I think we are talking about the same thing. It sounds like a broken converter. If you can devise a way to touch it without burning your hand, I'm sure you could feel it rattling around in there, if that's the problem. Maybe try it on a cold start or something.

 

I will give that a try in the morning. The noise isn't there when cold, still could be the converter you think ? thanks for the ideas thus far !!!

 

Its could still be the converter, that is where I would start.

 

So I tried crawling under the car this morning when it was cold and couldn't hear or feel anything on the cats, oil pan or transmission. I just had the car out for a while and when I got home the noise was continuing. I jacked it up and slid under the car and from what I can hear/ feel it definitely seems like its coming from the transmission. It will make the noise in Neutral, or in gear with the clutch depressed, or in neutral with the clutch depressed.

When driving the transmission and clutch both seem to be working properly, clutch grass good, transmission shifts easy and smooth ETC...

Any thoughts as to a cause or solution?!

Thank you again in advance!

 

I've got nothing beyond make sure it's full of oil. Of course if it's not then the damage is already done, and trans repairs are expensive. It sounds like something banging around in the bell housing, like a bolt or something, but that would be noisy regardless of if it's hot or cold.

 

Yeah that is what I can't get my head around is that it only does it hot. And besides the noise it runs and shifts good

 

Because it is a family member, I'd recommend that you both -- wheather you buy it or not-- put the car up on a lift and figure out that sound. If you faimly friend try to sell it to anyone, he will need to disclose whats up with the car.

As you said the noise is random so my guess (I agree with PDT) is that something is bouncing around in the bell housing. If so, 10 minutes on a lift and you will know. This could be an easy fix, but I'd probably not drive it (much) until you can have it looked at.

 

I had it on a lift once and of course it didn't make the noise the entire time, probably about 15-20 minutes, (maybe not driven enough prior to doing it) I will try to get it back on a lift again and see but am mostly sure its definitely from the transmission. I will have to talk to the owner and see what he wants to do, I obviously don't want to sink a ton of my time or money into it and keep chasing problems around. I definitely want the car if its worth it but just need to sort out this damn noise. Thanks again for the hints and ideas. If anyone else has any or has had a similar issue I still appreciate any feedback.

 

 

After more searching and listening that is what I am leaning toward the cause being. I have been speaking with Bill of ZFDoc the last couple days and he has been extremely helpful.