Something is quickly filling my macbook pro's disk!

I have recently been maintaining about 4 or 5 gigs of free space (as shown ‘available’ by finder). Today, I receive a warning that my startup disk is almost full! Every time i clear some junk files and make room, the space mysteriously is filled. This happens quickly, .2 Gig in the two minutes it took me to type this… What could be going wrong? I am running 10.4.11 on an intel mac, 2.2 Ghz core 2 duo

Try the Linux/unix/Mac forum
http://forum.avast.com/index.php?board=5.0