I have a humming noise that occurs sometimes at idle and sometimes when coasting. It goes away when I accelerate while driving but if I increase RPM in park it has no effect on it (if it's occurring, it comes and goes). The sound itself doesn't change when it's occurring, it's either there or not. It is also slightly louder in the cabin than it sounds if standing over the engine, if that helps any of you diagnosticians out there.

In searching I read about a guy with an S Type with a very similar sound issue. He ended up suspecting his EGR valve. I think it turned out to be an elbow on his throttle body somewhere. My most recent work on the car was to install a new throttle body, EGR valve and Evap Purge valve. Car drives wonderful, LTFT are between +2 and +7 on both banks.

Here is the "maybe it's important maybe it's not" part. The EGR valve was bought brand new for my previous XJR. Some yahoo totaled the car about 3,000 miles after I put in the new EGR valve. During that 3,000 miles I was starting to hear the same sound in that car. I didn't suspect the EGR valve so since is was a brand new Denso unit I installed it on my new XJR even though there was nothing seemingly wrong with the one on the car. I was in there doing work anyway, I have an almost brand new one on hand and new gaskets on the shelf, what the heck.

The EGR valve is not that easy to get to and if I removed it I'd have to order all new gaskets again. If this is a crackpot theory I don't want to go through all of that. Maybe it's the intake tube? Maybe it's the new purge valve? I was just hoping someone had some experience with this sort of thing. Thanks

 

The Evap Purge valve was also new on the car that was wrecked and was then transferred to this car. When I put the stethoscope on it just now after driving to the grocery store and back it seems the sound is indeed coming from that purge valve. Funny how the sound can seem to be coming from the top of the engine though...

I noticed today that the sound was in fact much louder in the car than outside so I suspected something at the firewall. The Evap Purge valve was humming. I have a used one on the shelf in my garage. I'll swap it out I guess and see if that makes a difference.

 

It turns out the noise was due to a ham-fisted mechanic, both on the old car and the new one. ME!

The two 8mm mounting bolts for the evap purge valve assembly were too tight defeating the rubber bushings built into the mounting tabs. I loosened them a few turns and the noise is gone.

I feel foolish but also happy it wasn't something more serious.

 

Jeff, don't feel foolish. We all have made these kind of mistakes and learned from them. Your post will help me from making that mistake someday and save me time trying to diagnose the noise.
Thank you for your post.