It seems like since it has been cold my 1998 GMC (Vortec 454) has had a very faint knock. The truck runs the same and has great oil pressure, but there is a noise that I hear only when idling. I can rev the truck and the motor sounds great. Now for the weird thing...the noise is the loudest (which is still faint) when I have the door open and am kneeing down between the door and cab. You can't hear anything under the hood. I can slightly hear it when in the back coming from the exhaust. It sounds like a bottom end knock to me, but it doesn't sound like it is coming from the bottom end. I have heard of catalytic converters making noise, but I wouldn't think it would be as steady as this. When I say steady the noise keeps the same rhythm. I know it is hard to diagnose by a simple description, but has anyone came across anything similar? Thanks!

 

I went to other forms , and searched. Read about your symptom, Most went too 10w 30
stopped it, if it is in bottom end. When you first crank it up , turn back off crank again see if it quits. Could be a cat , or loose heat shield. Might have to crawl under ther narrow it down. Good luck hope you figure it out.

 

I think I found the noise. I took the truck to the muffler shop today and had them put it on the rack. My rear most cat was sitting right on the crossmember due to the rubber hangers stretching. I had them jack up the exhaust and put new rubber hangers and haven't heard the noise since. I also had them put a new magnaflow muffler on and it sounds great - nice deep tone and not to loud on acceleration in the cab. Keep your fingers crossed for me that it was as simple as that!!!!

 

I take it no more KNOCK! Good i hate to crank one up , and hear that rod.

 

No more knock...drove it all day today and it is running great. I am really happy with the Magnaflow it sounds nice and deep outside, but is barely any louder inside. I don't know if it just me, but it feels like it runs better too. Now if I can just get rid of the stupid brake squeak!!!!