Cockpit noise/humming

[soupisgoodfood]

Hi. I had a chance to sit in the cockpit of an old 727 (looking at an aircraft maintainance course). They even fired it up for me (but not the engines). One thing that I noticed was that there's a very loud humming noise. It's constant (and high) pitch, and it seems as if there's more than one source, so it creates that funny effect you hear when there are two sounds of an almost identical pitch.

I'm guessing it's just some relays or inverters?

[ATCO17]

That'll be the static inverters producing 400Hz for instrumentation etc.

[vapilot2004]

ATCO17 has it right. There is also a whine from equipment cooling fans and on an old lady like this, multiple spinning gyros add to the harmony.

[Dan Air 87]

Yeah but isn't the sound of the three engines on take off something to behold? I always managed to bag seats at the back just to listen to that sound? It was fantastic and I loved the 72's that Dan Air operated-Oh what wonderful days! A superb aircraft.

[soupisgoodfood]

Thanks, guys.

Now I just wish my car would sound more like that. Do you think they'd notice if pinched a few of those components after my next flight?