Interesting electrical problem ...

1990 SN. A few weeks ago I noticed my Oil Pressure readings on the dash analog gauge were pinned to the right, maybe 70-80 PSI. So I replaced the Sensor at the engine with a new one ... no change after that, still reading high, perhaps this is unrelated ? Then I started to hear a very slight buzz from the alarm on the Dash, only when the engine is running and hardly noticeable, and my voltage light on the Dash was lit. The readings on my Voltage analog gauge were low, 10 to 11. So I tightened the ALT belt, which was loose and I did notice belt dust, and that seems to have taken care of 1-2 issues, the V light no longer comes on, the analog V meter is now reading between 12 to 12.5 with engine running, I would have thought this gauge should read in the 13's with engine running ? But the alarm still "lightly" buzzes ... sometimes. After a longer run at higher RPM's I do not hear the alarm buzz. It seems I only hear this when the engine is first started or after it has been idling for some time, and it generally goes away once the RPM's go up. So perhaps this is a charging and/or ground issue ?

When I hear this alarm faint buzz, if I turn the Horn button on, the horn does not sound but the alarm buzz goes away for as long as I depress the horn button. And at higher RPM's, if the alarm buzz HAS gone away, the Horn does sound when the button is pushed.

I also noticed my Oil Pressure analog gauge was flickering back & forth between 0 and say 60-70 PSI occasionally the last day or so, and if it went to 0, if I changed the RPM's, it would quickly go back to 60-70 PSI or so.

Both ALT readings at idle speed with my digital handheld meter have been 14.20 or so during all of this. Same readings on my Battery at idle. Each morning (after running the day before) I have checked my Battery before starting, and it is reading about 12.6 on my meter (battery is 2-3 years old). My NEG battery cable is connected to the engine block, and it is clean & tight. From there another 4 AWG or so cable goes to an engine mount/stringer alum plate, and a smaller maybe 10-12g black wire with ring terminal connected here and goes forward, I have not yet traced this wire. This connection is clean & tight.

Any idea what is happening and / or what I should do ?

Thanks, cheers, John