intermittent whistling

My 99 XJ has this whistle, its been getting worse lately.
It only happens after the vehicle is warm and it happens when i let off the gas, or when i am holding it at 1/4 throttle or less. The other day it did it as I came to a stop, and i got out and tried to listen ,it sounds as if the whislte is coming from almost teh back side if the engine block, i cannot tell which side. I searched and found a post from a while back that sounds similar to what mine is doing, and it was suggested that the cat might be clogged up
Does that sound right to all of yall? If so, how would i check, if i pull my cat, would i even know then? Maybe someone knows something else it could be,
I hope i can get this fixed, it gets realy annoying on the interstate,
Adam Spencer

check to make sure that exhaust isnt cracked somewhere first- get a friend with some good white shoes- have him put his foot over the exhaust for a second or two while you hear the whistle- if it changes, you got a small crack that only opens when the exhaust gets warm- favorite spot is right at the manifold where the pipes join, but can be almost anywhere from there to the cat....
when friend complains about white shoes being black an sooty, thank him.

the easiest test I know of for a semi blocked cat is a sniffer test by an emmision station- a blocked cat should throw off the readings even if the cars running good.

OK, i will check that soon, im in college, so i dotn have any room to work on it for awhile, but ill check on that. What would explain the fact that it stops if i push the gas? I would think that if it was a leak, the pressure that is created by having a load on the engine would make it whistle louder, instead of stopping completely, which is what it seams to be doing. Also when it was idling and i was listenign to it whistle, i reved it up slightly, the whislting stopped and didnt return, it seems really wierd to me.
thanks for the help
adam

I'd also check the vacuum with a gauge as well as inspecting lines, and see if there is anything leaking, especially if the whistle changes consistently with throttle position. A leaking intake manifold gasket might whistle.

intake manifold gasket, custom cone filter, vacuum system... Im a fan of spraying the vacuum lines down with a flamable airsol - when the engine revs, you found the hole...

an old thread I know, but I know the whistling and it's most likely source-- the plastic elbow/grommet vacuum line that goes into the brake booster!! I had this horrible variable volume/pitch whining at mid to half throttle (where we spend most of our time, of course) that I could hear loud and clear even while doing 70mph with the windows closed and radio at normal volume at night! ok just kidding night/day are irrelevant, but I really thought it was a tps related electrical rheostat(sp?) or something similar gone completely haywire until one day when my voltage regulator went bonkers and started frying my battery with 16+volts and a friend (you know, the electrician kind with the really nice multimeter and I had the hood up and were testing the usual suspects and in my naivety I told him that I had a crazy, high pitched "electrical" whine... he checked around for about 20 seconds or so, wiggled the elbow/grommet on the brake booster and bingo!

well, shortly after i posted the original topic in this thread, i went ahead and changed the intake manifold gasket, figured i was only out $14 and a couple hours work if that wasnt it

the gasket i got at the dealer was much better quality than the sheetmatal one i took out, and i could see on the old gasket where it had been leaking,

fixed the whistling, and it hasnt come back since december

adam spencer

oh, ok- well did you have fun tightening those bolts underneath?

pbandj said:

oh, ok- well did you have fun tightening those bolts underneath?

Click to expand...

haha, yep, i actually got pissed and pulled the front shaft, i have long arms, and just reached up, once i thought to do that, it went pretty smoothly

adam