Strange Knocking sound? Help?

Hey Ya'll

I'm noticing a "knocking" sound and it's coming from a weird place: the passenger compartment floorboard/firewall area. But here's the thing; it only happens at idle (goes away around 1500 rpm), and you can't hear it under the car or in the engine compartment!

The frequency of the knocking increases with engine rpm, but I really don't think it's an engine knock; Getting on the highway this morning, I took it to 5,000rpm in first and second gear, and it was as smooth as a babies bottom. Could it maybe be in the bell housing, like something with the pressure plate or clutch?

Anyone have any idea what this could be? I really hope it's not internal engine, but if it is, I need to figure it out fast!

Thanks in advance!

I was going to say flexplate bolts, but you mentioned a clutch.
Can it be heard with a stethoscope?

Yah, but it's not a bad point, I could have pressure plate bolts or something loose too...

Good idea, I have some tubing to rig one up, i'll have to check it here in a few minutes...

Maybe the exhaust? My wife's 91 need a new muffler and it makes a knocking sound only at idle from the passenger side.

If it's a later model (97 up, I think), check the evaporative purge valve. It's near the passenger side firewall attached to the evap system lines. There's an electrical connector on it, so just unplug and see if it changes. Mine (99) knocks at idle, and it sounds just like a bad connecting rod, though it doesn't seem to change speed with engine speed.

Also look at the exhaust. The clearance where the exhaust goes over the crossmember is very small, and a sagging engine mount or a bad exhaust mount will cause it to knock at idle. Usually, though, that's an irregular, rattling knock rather than a regular one.

Thanks guys, I won't have time tonight anymore (spent it changing the rear diff oil) but i'll have to check it all.

I know it does have an exhaust "tick" but that's up by the manifold, and the funny thing is this knocking sound can't be heard under the jeep???

Hmm, I couldn't see any sort of valves down there, but I only looked for about 5 seconds today haha

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A look inside my 99. The purge valve is the small black canister just to the left of the light green cap (not the one with the plug already pulled - that's the A/C, which I always pull in winter so I get hot-only defrost).

My exhaust makes a rattling sound only at idle and only when cold. It is metal to metal contact between the exhaust and the body. Fire it up and crawl underneath.

I can't believe it, it was that black EVAP valve! I can feel it in my hand and I confirmed with the "stethoscope" method. I unplugged it and it stopped. I got the ses light though, so I plugged it back in because the light annoys me more than the "knock" haha

Thanks again!