Guys,

This is a weird one, but I'll give it a shot. I'll preface it by saying that my 540i has become a rarely driven weekend warrior (long story...), which is to say, I don't drive it more than a few times a month, usually on longer weekend trips. I keep it on a battery tender...

Anyway, the car makes this noise only when the transmission is in (D) drive, not in (4) sport mode, which leads me to believe it's probably something with the overdrive gear. I've got about 101,000 miles on the vehicle right now.

The noise materializes only if I'm driving at a steady cruise speed, and the engine becomes unloaded - i.e. I back off on the accelerator very slightly, or I reach a section of road that is slightly downhill.

The noise sounds (I don't have any other way to describe it) like someone is spraying a high-pressure jet of water against the bottom of the car. Even if the right conditions exist, the noise doesn't always materialize. When it comes, it lasts only for a second or two. Sometimes it comes back again after a few second break, sometimes not. The RPM's don't fluctuate while the noise is going on, nor is performance of the car affected. I'm not showing a "check engine" light.

As nearly as I can isolate it by ear, the noise is under the middle-right side of the car - about even with the transmission shifter, but on the passenger's side.

Can anyone offer any ideas/experience/suggestions before I spend the money on professional diagnosis?

Thanks in advance!