Dear Tom and Ray:

I have a 2009 Mini Cooper that has a weird, intermittent problem. When I am idling -- like at a stoplight -- I hear a slight bump underneath the car. It's as if a mechanic has rolled under the car and sat up, banging his head on the undercarriage. And it comes every 10 seconds ... exactly ... like clockwork. The really weird thing is that after I turn off the car completely, it will continue for roughly a minute and then finish off with three bumps: "ba-dump-bump." It's all very soft and hard to hear, but I can hear it. What is it?

-- Charles

RAY: I know exactly the sound you're talking about, Charles. It's like when my brother slides under a car to take a nap, and then wakes up and hits his head. Only with my brother, he stirs every 10 hours, not every 10 seconds.

TOM: You know how dealers often will try to get rid of you by saying, "They all do that," Charles? Well, they all do that.

RAY: It's part of the evaporative emissions system on this car. All modern cars have a system that traps gasoline vapors so they don't escape into the air and create smog. What you're hearing is an air pump that's part of this car's self-diagnostic system.

TOM: When the car is at idle, every 10 seconds it pumps a burst of air into the evaporative emissions system and then tests the pressure to make sure no vapors are leaking out. And yes, it even continues for a minute after you shut off the engine, because the system remains under pressure for a bit.

RAY: If the sensor in the system were to detect a drop in pressure, it would turn on your "check engine" light, letting you know that there was a problem.

TOM: In fact, it's called the DMTL pump. Mini says it stands for "Diagnostic Module Tank Leak." But I think it's "Don't Make That Light" come on!