My coworker noticed this twice within the last 3 months. It happened both times after the car sat for about 3-5 days without being turned on with the exact same results.

Upon starting, there's a knocking noise but not really loud like engine knocking, maybe closer to a rattle/click (yes I know how dumb it is to try and describe a noise in text but here we are anyway haha).

What happens then is as the car gets warmer it just gets more quiet and dull and finally goes away completely. This noise doesn't exist if the car is turned on every 1-3 days and the engine sounds and runs perfectly fine otherwise.

We think it may have something to do with cracked headers or the aftermarket cats previous owner had installed.

Any ideas?

 

seek professional help, not a forum...we cant hear your click, dont ruin your engine..

 

Valve train noise? Get it diagnosed but you describe what I used to hear in an older car. Especially when cold and/or left sitting for a few days.

 

What engine oil are you using?
If unsure change filter and oil with 0W-30 synthetic.
Will.improve oil flow on start up.
Do not use 0W-20.
Replace ATF in power steering system with synthetic grade ATF.(One turkey baster extraction at a time)
Search these forums for info on ATF
Remove serpentine belt to see if noise abates.
Inspect serpentine belt for cracks.
Spin tensioner, idler, fan bracket, AC and alternator and tensioner pulleys for noise.

After the easy stuff, then look to other areas.

 

Use a LOOOONG screwdriver as a stethoscope against your ear and points on the engine. But be careful. My #7 valve used to click on my 93 and now the same on my 94. But a loud click could also be exhaust leak. With the stethoscope you can pinpoint the area a little where it's coming from.

 

What brand oil filter do you use?
You didn't specify the time length of the knocking noise nor provide any actual sound, it's hard to judge. But if it lasts a few to several seconds, there will be a possibility that your oil filter is not working right. A poor anti drain valve

anti drain valve

used in the oil filter causes a knocking noise when starting up after the car is sitting several days or more. Some filter manufacturers don't use a good material for it and that causes the knocking noise. Even a genuine oil filter will cause it if you don't change the filter for a long time.

 

I vote for deanshark, we used to use that method in my shop, many years ago and I just fogot..