2000 Grand Marquis, intermittent noise

A loud humming (sounds like a compressor). I can place the car in neutral, the engine revs up, and humming will stop. I have to repeat this action for the first 30-45 minutes from start of drive time.

Sounds like the IAC (Idle Ar Control)
When mine failed, there were no drive ability issues at all. Gas mileage was fine, drove great, just the noise.

Hey, I just thot about it. I had a '99 Lincoln that had air suspension on the rear. The air would leak down and the compressor would run the first time I started it each day. Occasionally the compressor would come on to adjust the suspension. Betcha that is the sound you hear.

Right…Many Grands have air-ride suspension and a compressor right up next to the steering box…When the air-springs start leaking, the compressor starts running more often…If this is indeed the problem, you can replace the air-springs with steel springs without too much expense or trouble…

Of course, if the air holds up most of the day, just leave it alone.
I an not sure, but I think the steel springs are there.