Have had a whistle noise from under the hood that is present at idle and changes pitch with increasing throttle..... thought it was a vacuum leak but my mechanic says it's coming from the alternator as he thinks the bearings are going..... sound plausible?

 

Very possible. But it can also be any of the other pulleys too. The best way to check is remove the belt and give each pulley a spin by had also see if any of them have extra play. They should spin smooth with out any slop to them.

 

X2^ Also a bad bearing will get hot, (at some point). You might start it cold, run it a few minutes then turn it off and check for heat. Also you can listen with a hose or tube. Good luck!

 

If you don't want to worry about taking it off, just listen with a hose or a mechanic's stethoscope (approx. $10 at Harbour Freight/Princess Auto). If the bearings are bad on a pulley, it should be pretty obvious.

 

If it does turn out to be the alt, an auto electrical shop can replace the bearings and brushes in short order, ~ $45 here but I assume a bit more in Boston. My bearings went about 5 years ago, pretty much completely locked up but making very little noise amazingly enough

 

Otherwise, you can just get a new 100A alternator for around $100. Both good options, if it was me I would replace it if it is an old alternator. They are ridiculously easy to replace on the XJ. (unless perhaps you have some skid plating that would interfere).

 

I agree... Advance Auto has em and with on-line coupon codes it'll be under $100..... thanks.