93 jeep cherokee
4.0, 6 cyl, 4x4, automatic tran, 220k mi

tends to happen when idling.
just did an oil change and replaced spark plugs doing tune-up to prepare my toy /wifes daily driver for summer. trying to fix this knock as well.

 

the knock/noise is worst with ac on.

suggestions?

 

seems rotational like something connected to the belt. When giving it a bit of gas I wonder if it is the speeding up making it do it.

if not, it could be something broken or vibrating as it is very rythmic

 

the tapping/knock/noise goes away when engine is revved.

 

possibly...possibly your flex plate, or maybe your exhaust is rattling. Check your motor mounts, if they are bad you may have something knocking when the engine vibrates more. That is the easy stuff to check, you can go from there if you have to.

Edit: forgot to watch the second video, I have no idea why the AC would effect anything I listed above, sorry :/

 

I second the motor mount idea... That's about the only idea I can think of with that kind of sound...

 

my motor mounts look horrible, ill start there. thanks guys

 

Father-inlaw's 95 had bad motor mounts..... same 'mystery' knock.

 

put ur ear on the oil pan, could be from in there.

 

listen to the exhaust and have some one else rev it (dont stick your ear infront of the exhaust pipe, just next to it near the exit), and listen if you can hear the knock from there, if not then it is probably not internal motor knock, i second the motor mount idea

 

It sounds like detonation to me.

 

I've got the same knock. It was as bad as yours until I redid the exhaust and I still have it at startup but it stopped rattling at idle now.