About two weeks ago my 90 Volvo developed this low rpm rattling or squeeking or tumbling,some strange noise at a low rpm range: about 1500 to 2000 RPM.

I picked up a mechanics stethoscope and heard some sounds from the alternator, I removed the alternator and swapped in a rebuilt. It turns out the sound is still there. (the alternator bearings are indeed worn, but it wasn't the problem).

The noise continues, but never at higher rpms. It's hard to tell where the noise is coming from, even with a stethoscope. I know it's not the valves.

These "where is the part with a problem" questions are hard to answer, I am trying to compile a list of suspects. Any input is appreciated.

Should I just ignore this until something happens?

Viva la Volvo 240 !!