About 2 months ago I noticed a slight ticking from the engine when driving and applying a little gas. At first I thought it was "pinging" and was going to fill up with premium gas next time, but then changed my mind. This sound was regular and not quite as high pitched and "scratchy" as pinging, and it happened at only light loads. My next guess was tappets, but this was no different hot or cold and was not there at idle. From inside the car, in neutral, I didn't hear it, but in Drive, with the brake applied, I started to hear it past about 1200 rpm. Originally I tried to identify it by opening the hood and revving the motor by turning the throttle, but the general engine noises were a lot louder and, obviously, since it wasn't under any load, I wouldn't have heard the sound I was looking for anyway. I can't check it that way under load by myself, because the hand brake doesn't work.

I've searched here and couldn't find anything that exactly matched what I'm hearing. What I want to know is; what is it, should I be worried and how do I fix it?

The car is a 1992 745 NA wagon, with 478,000 klicks on it. Timing belt and water pump replaced a year ago.