engine noise

don't mean to make my first post an annoying "tech" question, but here goes. (Oh, I'm a member at jeepsunlimited and thats usually where i go for my forum needs.. but decided to sign up to naxja when i ran into this problem)...

I am having a wierd engine based noise on my xj (88 4.0l 5spd). It's kinda hard to describe, but it is in sync w/ the engine (i.e. get's more rapid as rpms go higher). it also gets to be a slightly higher pitch (but it's still a farely low sound). it sounds like some sort of exhaust leak, or something. not sure. i checked for holes.. didn't find anything.. retorqed the exhaust manifold. i can't seem to find anything. it just up and started about 2 weeks ago out of the blue. it does sound like exhaust or intake. i just don't have the good ole ear developed yet. it seems to happen more when engine is under pressure (when i am giving it some gas) although i can still detect something when just idling. any ideas of what to look for? i wish i could describe the sound better, but words aint coming to me. don't laugh at my descriptive terms, but basically, what it sounds like to me, is, a noisy exhaust leak that "hums" along w/ the engine firing.

Could it be the guts of your catalytic converter beginning to get loose inside the case?

Hums...Taps...or Whistles?

If it's a hum I wouldn't really think exaust or intake, maybe a bad harmonic balancer that's rubbing (or causing the serp. belt to rub)

if it's a tap I'd say exaust. Probably a cracked manifold (very common), you can also try to tighten up the flange (where the manifold bolts to the downpipe). Also, drive though a puddle, the noise may get MUCH bigger than go away entirely (easy exaust fix that I've done!).

If it's a whistle...Intake, check all lines and connections. Have a friend gun the throttle a little while sitting there and try to find it (mind the fans and belt though!)

Sequoia

i went out and banged on the cat w/ a rubber hammer.. nothing rattling around or anything. i highly doubt thats the prob though.. everything past the exhaust manifold is relatively new... plus the sound does seem to be coming from the engine bay.

hum is prolly the wrong word... not really a hum, jsut can't think of a descriptive term to describe it... it's not tapping or whistling either... it actually sounds like all the engine noises have just been amplified greatly...

Check carefully for manifold cracks and leaks. Another one that had me stumped for a while on my 87 was a rusted out EGR tube. If you recently replaced exhaust, check for loosening or misalignment of the headpipe to the manifold too.

You might try a stethoscope, or a "shade-tree stethoscope," i.e. a length of tubing stuck in your ear, and see if you can zero in on the location.