Tapping engine while in drive or reverse, at idle only?

[SWICKENSJR]

I bought a used 2007 Toyota Yaris with 52,000 from the dealership last week and was enjoying the sound system a bit too much so I didnt really notice this noise until a couple days afterwards.

The car runs perfectly, gets excellent gas mileage, no CEL or SES lights, engine oil is good, and no driveability issues that I've noticed whatsoever.

But I noticed when I parked in the structure at my complex I heard a tapping noise echoing off the wall the I was parked against...It's only when the car is in drive or reverse (at idle only, the noise goes away when I accelerate) and disappears when it is in neutral or park. It's not very loud (but easily heard from the echo) and cant be heard if there are other cars around but if i shut the radio off and it's completely silent I can hear this consistent tapping noise coming from under the hood, sounds like it may be on the passenger side. It doesnt sound like metal tapping and I checked to make sure nothing was just rattling against the engine but I couldn't tell.

Does this sound normal and I'm just being paranoid or what?