I have a 97 honda civic with auto transmission. I noticed today when I reved the engine that when I let off the gas the engine made a very slight grinding noise. I was in park so I don't think it would be the transmission. While driving the car I have had no problems at all. Any ideas on what may cause something like this?

The good news is that it's not the engine. Otherwise your engine wouldn't be running right now!

Reving the engine puts a lot of stress on the A/C unit if its on. Also look at power stearing or alternator. Check belt tightness and clearences.

Start the engine, pop the hood and listen and see if you can pinpoint where it's coming from. See if it's coming from the distributor... there's a bearing inside that went bad on my Civic '90 about half a year ago, and it produced a horrible screeching noise.