BMW 1990 535i: 3 beeps

From time to time (bet. 25 and 45 minutes) the car emits 3 beeps that seem to come from the dash, left side.
Also, every now and then, the check engine light will turn on and last bet. 30 seconds to 1 minute.
2 weeks ago, the car was shutting off after warming up, so the mechanic had me change the fuel filter and the fuel pump.
What can the 3 beeps mean? I really thought changing the fuel filter and pump was going to fix it.
Why the interior lights, that used to turn on when I open the doors, do not any more, even though the bulbs and the related fuses are OK?
Thanks.

Normal operation:

3 beeps is a radio with a face plate that can be removed, that is powered off with the plate attached.

Later,

The interior lamp/beeping problem could be the Interior Lamp Timer Unit, they fail causing the light problem and also signal the warning gong under the left side of the dash. Next time you hear it, try to zero in on the noise location, aftermarket faceplate warnings usually only occur after key off.......more info available if needed.

Thanks ...
The 3 beeps are no more. All I did was fixing the CD changer.
But the interior lamps still do not come on. Where is the Interior Lamp Timer Unit located? Where to find it?

I get confused at times, E34's don't use a 'timer', the interior lamps are controlled by the General Module when you open and close the doors. IF they don't work at all [manually using the dome lamp switch] check F18 & F21 in the front fuse box. F18 powers the rear 'map' lamps, F21 powers the dome lamp and 'map' lamps.

I replaced F21 and the interior light problem got solved.
Thanks

At times, the engine makes noise as though it's getting weaker, especially going uphill. Could it be the gas? Some people say it's because of the ethanol in the gas.
What is the best gas for a 1990 BMW 535i? 93 OCTANE? 87 OCTANE? or 89 OCTANE?

Personally, I've used Chevron Premium for a long, long time.

I don't think we have Chevron in New York.
Could it be that the valves need to be adjusted, like old european cars with manual transmission? This tranny is automatic though.

Hey,

I Just Did Brakes. I Replaced The 4 Sets Of Pads, 4 Rotors And 2 Sensors. I Still Get The Brake Linings Error Message In The Dashboard Screen.
Can I Reset It Or Do I Have To Bring The Car To The Dealer?

Thanks!