possible grounding issue with a stereo component

 

I have the same issue in my 97 XK8, however I purchased it without realizing it had a ton of after-market amplification installed. I'm pretty sure thatit is the cause of the sound, and I have found a loose wire behind one of the rear speakers, but I'm not clever enough (or have enough time) to correctthe issue just yet.

 

by the description I will almost guarentee its a grounding issue, but to be honest i don't know the x-type's setup. I know a poorly grounded amp can cause what you have described but iirc the 2003 x-type did not have an amp, just the 04+, but it sounds like an audio issue. fluctuating with the RPM would be interference from the alternator I believe and that is still present with the radio off, but to make sure its from the speakers why not have someone put there face right next to it and see?

 

well the grounding issue that I am thinking of still gives interference to the speakers without a signal from the headunit, this produced a "whine" even if the input was off. there is a chance that the problem here is different though

 

see that is what I always thought until I actually had this happen on a car I was working on. after doing an audio install, a "whine" such as what I think the op is saying was apparent. Talking to a friend of mine, he said that it was the ground for the amp, but it didn't make any sense as to how that could be, so I dismissed it. Maybe 2 weeks later, the next chance I had to work on it, I figured what the heck and changed the ground for it. After checking to make sure it was grounded with a multimeter and being satisfied, I started her up and all was good.

So there is still a good chance that this is not even close to what is going on with the op's car, but surprisingly enough, radio off, w/e it can still happen. Disconnect the harness and it shouldn't though.