I have xj8 4.2 (85k miles) and when I first start it (when the engine is completely cold), it has a knock sound. Just like someone knocking on the door. The sound is really louder from underneath. If I rev it up, it goes away and once the engine warms, it goes completely away.

Is this a bad connecting rod bearing? That is what is sounds like. Why would it stop doing it once it is warm? What else could it be?

 

Solved my own problem accidently. I had just done an oil change and put a Fram oil filter. Then I read that Fram filters are the worst, so I switched it out to a NAPA Gold Filter (Wix). Guess what, no more cold knock. I guess it had something to do with that filter and the oil pressure at startup and cold run.

 

Very interesting...

 

Sounds like a faulty drain back valve or lack of one in the filter.