High pitch component to idle when warm

[greggarrison]

1989 Volvo 244 DL (B230F I believe)
160K miles, Automatic

Recent Repairs/Mods:
1. New fuel shut off relay, other fuel system apparatus (O2 sensor, etc)
2. New transmision mounts and motor mounts (AT fluid ate the old ones)
3. New rear main seal & bushing for transmission

Problem:
When the car is up to operating temperature, as indicated by the tic mark on the temperature gauge, a high pitch sound can be heard coming from the engine compartment. It is in addition to the normal engine sounds, not replacing them. It sounds a lot like a power transformer does--high pitch whiring or humming. This sound does not happen when the car is not up to operating temperature and begins percisely when OT is reached. If the temperature drops, e.g., from idling while parked, the sound stops. (The sound happens whether the car is parked or moving).

Background:
The car just had the last two repairs (2 & 3) done two Fridays ago. It also had its transmission fluid changed at the time. No unusual sound until yesterday. (Saturday). On Friday it was driven hard (75-85 mph) on the highway for two periods of about 40 minutes and also in hilly terrain later on. Then the sound surfaced. The car also has a long standing problem with its climate control system: the heat never really shuts off and thus the passenger compartment gets hotter and hotter the longer the car has been running. To compensate I used the air conditioner for pretty much the first time on Friday. Might the sound come from it? When it is shut off by the switch though and sound continues.

Troubleshooting:
I checked the oil and it is normal. It was just changed.
Water/coolant is normal although resevoir seems unusually cool.
I also checked the AT fluid and noticed in smelled kind of burnt. Transmission is shifting fine though.
I tried to find the source of the sound but could not.
I happened to observe the engine at night in the dark and noticed that there was some current leakage from one of the spark wires onto the block: it was arcing a little onto the block. Still seemed to be firing on all cylinders though.

Is this noise a symptom of serious problem? It is annoying in any case.

Any ideas?

Thanks a lot.