96 Honda Accord possible tranny problem.

I have a 96 Accord LX (auto tran) with about 150,000 miles. When I back up (especially if its going up hill a little) I hear a creaking/grinding noise and when I'm reducing speed and my car switches out of 4th into 3rd gear I hear the same creeking/grinding noise. Theres no problems (at least not as of yet) from 3rd into 2nd or 2nd into 1st. Can someone tell me whats going on? It's been like this for over a year. But it has been more frequent since mid-summer this year. (Before it was just an once a couple month thing, now its about once a week or every few days. If you guys can help I'd appreciate it. Thanks in advance!