When I have the key turned to the run/on position I hear an odd low pitched humming coming from behind the air vents. Anyone else experience this?

 

just the fuel pump more than likely...

 

I'm assuming it's nothing to worry about. I bought myself a crystal red pearl '05 limited roadster on 12-21. Yes, I got an xfire for xmas. 42k

 

Depending on what vents you are talking about, Mercedes put blower motors in the front SAMM box and the passenger floor board compartment to keep the electronics cool.

 

Without the heater/AC fan on?

 

If you don't have the radio on when you first start it up you can hear the secondary air injection pump running for a minute or two. It is kind of a high speed whine comming from the front of the engine. I can hear mine inside when I first start it.

 

It's this. Sounds like a vacuum cleaner. Mine does it until the car warms up a little.

 

Ah, hell, you get my age, you don't hear any of that stuff. My cars are quiet as a mouse....lol

 

Maybe it's hmming a Xmas song as a thank you for adopting it and giving it a good home.

 

This is quite possibly true

 

Most likely the air pump - it does sound like a vacuum cleaner.
Mine runs for about 1 - 2 minutes after start.

 

The fan in the ECU/Relay module box started to emit a high pitch whine on my car back in september. I was able to, somehow, quiet it down by moving it(rotate) within its rubber mount. You might want to try that sherwinagain

 

The humming starts with the key in the "ON" position. the car has not been started. That's what I read in the OP's statement.
So it is not the air pump.

 

Agreed - scratch the air pump.

 

Actually as you can see here he says run/on so the pumps back in the running I'd say.

 

You are a brave man, Larry. When Dave tells me I'm wrong, I just say "yessir" and bow my head.

 

You may be perfectly correct, but ..... he say says 'turned to the run/on position', he actually never says it was turned past that point and the car started.

I would say the car was running not the way he says it. That's my interpretation, everyone has different ways of saying things, crank, turn over, etc. Shades of gray I guess.

 

When you put it this way Dave it puts it in a new light and I would tend to agree. I guess we need to know from the gentleman as to whether or not it was actually running or not.

 

you guys....I am sure if he started it, he wouldn't hear a hum.... He turned the key to the on position...and heard a hum....I can't hear anyway, so I have one quiet car....glad I have gauges so I know I can go....

 

I don't mean to be brave or contrary, as Dave says to each his own interpretation. That being said I respect Dave for his knowledge and willingness to help anyone that asks.