I have a Rheem Classic 90 , 3 years old . Started my a/c unit today and it made a noise at start up only . It was something like rrrrrrrrrrrrr, for a brief second , stopped and ran just fine after. Anybody have any clues what this might be.

It could have been a slight lack of oil/lubrication on start up. When they sit all winter it all drains down to the crank case and not much is free flowing to lube the upper end of the compressor.

One possibility if you have a line set that travels vertically to an attic liquid ref. can migrat back to the comp and cause a hard labor upon initial start and the lack of oil flow because of season start up.

The unit is probably equipped with a scroll compressor. Long periods of non-use (like winter) will result in noisy initial start-up, as the scroll is in the top and the oil is in the bottom. I wouldn't be concerned if this is a seasonal one-time occurrence.