I noticed that when the heat is on, it makes a hissing sound. more annoying, when i accelerate from low speeds, it actually groans as if it's in unison with the transmission - more groaning with more throttle, until i get to a certain speed, like on the highway. then i still get hissing. this isn't A/C; this is heat, though the auto button is what i press to turn the heat on in the winter. i turn the heat off under these conditions, and the noises stop and the car sounds normal. has anyone had this issue? is this a regulator issue, something easy to fix myself?

99 E320, 108K.

I too am hearing a "hiss" with the heat on. The sound reminds me of when the brake pads get too low and that indicator starts rubbing on the disc.

I'm anxious to hear what this is.

hey rd - do you also hear the groaning i'm hearing when accelerating at low speeds...? it's definitely not happening when the fan is turned off, which seems strange...

bump, anyone else have ideas or experienced this before?

Are you sure it is not the blower motor. Does this happen with the AC on?

i'm not sure of anything; asking what it could be? it's too cold lately to run the AC. i'll try it later but it's happening with "Auto" on as well as the directional mode which doesn't automatically shut off. if it's the blower motor what am i looking at in terms of a DIY fix or parts to buy?

No groaning, only the hissing/squealing. Not sure about A/C as, here too, it's heater time.

update, noticed today it happens with the regular vent/heat on, but it goes away when i turn on recirculate air. ???

zx11, if it is the blower motor, what can be done?

With recirculate air there is allways high sound.
Is the sound still there when you turn off recirculate air?

nope, it's the other way around for me.

with recirculate air, the sound goes away. with regular heat coming into the cabin via the outside through the cabin filter, the noise persists.