metallic knocking from rear wheels?

[mellow_sparky]

I noticed today (as my wife pulled away from the house) that there was a distinctive metallic knocking sound coming from the back of the vehicle. I checked all the lugs - they were tight.

So I called the dealer and the service guy said, "oh yeah that's the brake booster building up pressure at low speeds". True enough the sound goes away once you get up above 20 mph or so.

So, um, does that sound right to you guys? It's a 3.0si with sport. I feel compelled to take it in anyway. It doesn't seem right to me.