One thing that bugs me about my car is that the fuel pump noise is fairly inconsistent. Turning on the lights, aircon, defroster, etc. lowers the fuel pump speed and gives a lower pitched noise. It also dips for a second when the RPM gets too low, sometimes when accelerator is released and clutch is pushed in, sometimes apparently for no reason at all. I had times when engine RPM would be critically low immediately after starting until the fuel pump gets to full speed (it works slower for the first few seconds on these cars for some reason) and this inconsistency might be the cause of some intermittent idle problems I have.
Is this normal behavior? I do understand that the alternator is being spun by the engine, but it should have a voltage regulator and even if generated voltage dropped, as I understand the battery should maintain system voltage above 12V (battery works great, by the way). Shouldn't constant voltage always be provided to the fuel pump?
The car is a 1999 Volvo V40 T4.