engine noise

[vladthedog]

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.

[Eagle]

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

[GSequoia]

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

[vladthedog]

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.

[vladthedog]

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...

[Matthew Currie]

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.