Give some gas and it gets silent (or at least is inaudible), let off and the sound is back. It is fairly loud and easily heard with windows cracked open even a little, but not loud enough to hear with the windows closed.

At first I thought it was just rain/moisture in the belts/pulleys but then I noticed it immediately after starting up after 24-hours of being parked indoors (garaged.)

Any ideas?

How many belts/pulleys are on this car that fluctuate with throttle and move at around the speed of a cooling fan? (since that is the approximate tempo of the noise)

 

Belt tension arm bearings would be a first guess.

 

I concur, Tensioner idler pulleys. You can pretty easily remove or ease the tension on the belt while running. The older idler pulleys were smooth and make more noise that the ridged, double ribbed belts.
I have heard the A/C compressor idler bearing can also make a noise
I presume it is a squeak/rumble type sound. So hard to type a sound..

 

Could someone kindly point me to a photo or an engine-diagram of where the tensioner idler pulley is and how one would adjust it? How many belts and tensioners are on this car? My last car used a serpentine system that was complex but easy to trace by just following the single-belt path.

lol, it is hard to express a sound typed out, but it is not rumbling at all, just a high-pitch continuous squeak-wave that has the tempo-sound of something rotating about 3-5 times per second.

The actual sound does not bother me at all, just the concern for what it could be a sign of; is a noisy tensioner idler pulley a potential mechanical problem or can this safely be ignored?

 

You need a stethscope to listen to the 2 pulleys, or a long screw driver works too. With the engine running the tip in the tensioner pulley bolt and idler pulley bolt to listen to the noise. You cannot adjust anything. The bearing get dry and old like anything else(theyre sealed) personally i would replace the belt and both pulleys. The belts get glazed patches on them that squeek when they pass over pulleys and the bearing squeek. You can spray lube down on the 2 pulleys to see if it stops momentarily, but its not a long term fix

 

Thanks, so that means there are only two pulleys and one belt as potential problem source? I assume these are at the front of the engine block right? If it turns out that the belt does not have signs of wear, can I safely just ignore the issue?

 

Here is what the front of the engine looks like, at least an AJ27 NA. Not sure you identified what year and model you have. Again, do not know you MY and mileage but if I was going to the trouble of replacing the idle/tensioner pulleys I would replace the belt also.


edit:
[sorry, that is an AJ26 engine but someone had put into my library as an AJ27]

 

Okay, a mechanically-inclined friend shot some belt-squeek-stop spray lube and the noise went away for about 2-minutes, lol... He ended up shooting it under a few different pulleys (while the engine was running) and the noise ceased instantly after each pulley, then started again a few seconds later from somewhere else.

Either the spray just had no penetration and rubbed off or all my pulleys are squealing?! Does that make any sense?

Do I really need to change them or can I safely ignore the noise?

 

Changing the belt is really very simple and an obvious solution to your problem. You could also test the idler pulley bearing with the belt off. It is a common problem.

 

hope this helps you to identify the bits your looking at!

 

Regan, that is very helpful, thanks! Now I know about the replace-belt indicator too!

TestPoint, can you rotate all the pulleys freely with the belt off without issue? This is not like the timing belt right?

 

With the exception of the engine crank shaft all should freely rotate. I believe that you said that spraying the belt at any pulley caused the sound to go away for a few minutes. That screams BELT to me. While you have it off you can spin each pulley but the two idler pulleys are historically the most suspect. Alternator next. You won't be able to tell much on the water pump, PS pump and AC compressor as they will turn but not 'spin'.

With the history of the idler pulleys I would go in prepared to replace them or at a minimum disassemble and grease the bearings. Several have had success with that for some time.

Not like a timing belt, no alignment required.

 

Thanks - is there any specific belt to buy? I saw 7-ribs in the spec, but how long and is there any other criteria?

 

Jaguar changed from ribbed to non-ribbed or vice versa at some point. The idler pulleys are not compatible if you try to change the belt type. You should be able to buy the correct belt at most AP stores based on your MY and car.