1998 Volvo V70 transmission noise

In the last week or so my automatic transmission will make a tapping sound almost like something is slapping the bottom of floor pan when it is warm, in drive, and stopped. If I shift into park or neutral it stops making it. It is only at when I am, stopped like at a light or something similar. I checked the fluid, everything is normal. It hasn't been flushed since I got it, but I have only put 20K on it or less, so I was hoping it just needs a flush. But I was scared that might aggravate it? Or if its not it, that it'll be just money out the window.
It will now occasionally slip on a hill, but it is other wise running ok. I want to get on it before it gets worse.
It almost sounded like the noise a trans makes if you put it in park before it stopped moving ( don't worry.I don't do this, but have heard it on another car before).
TY