engine noise

My wife has a 97 4x4 with the 1.8 (I think) with about 145Km (about 90K miles?). It has been trouble free with reg oil changes and most recently timing belt + serpentine. It has recently started making a noise from the engine I can best describe as a ringing and grinding sound. I would imagine it is what a rock would sound like rolling around inside a bell, but not consistently. It seems to come and go when driving. I was able to hear it only at idle in the driveway and it went away with a little gas, and came back when returned to idle. Any ideas what this may be or how to isolate it? Thanks.

Im not that sure...but try taking a look at the teeth on your tranny gears

Thanks for the reply. It turned out to be the timing chain, diagnosis confirmed by 2 mechanics. I obviously need to quit working on my 400 Ford and learn something about my wife's engine. :) Kinda mickey-mouse in my book though it needed a new chain at 90K...the oil has been changed religiously.