What's that clunk when I turn on my receiver?

When I power-up my home audio receiver, it takes a second or two to warm-up, then there’s a mechanical clunk/thunk noise. It’s not from the speakers, it’s from the actual unit. The same thing happens when I turn it off.

Any ideas what this is?

Perhaps the transformer in the power supply?

It’s a relay inside the receiver. Mine is more of a click.

Probably one or more relays.

The amplifier’s outputs are usually connected to the speaker terminals through relays on modern equipment. This gives the receiver’s CPU control of the speaker connection, which allows the unit to do a few neat tricks. On some units, you can set speaker A/B/both/none options from the remote. The unit may be able to protect itself somewhat from accidental shorts by disconnecting the speaker terminals. And, as you have noticed, it can mute the outputs on power-up, preventing the annoying “ker-WHUMP” that would otherwise come through the speakers as the amp came on-line. This last item has become especially important with the recent proliferation of subwoofers.

It actually is mechanical in origin. An electromagnet causes a metalic arm to actually strike a contact - that’s what you’re hearing.

Yep, forgot that many receivers employ relays.

What I recall from my rudimentary electrical engineering course is that a change in flux results in an induced current. Turn on or turn off your stereo, and you get a “bump” of noise because of the momentary induction surge.

Or perhaps I misunderstood the principle entirely. Just thought I’d point the thread in a different direction.