You know that little screen / filter on the front of the enclosure? I once took it off to blow out some dust on the fan and dropped it on a nearby table. When I went to put it back - I discovered I needed a dustpan!
(we had just done some remodeling, and while I thought I had done a bang up job of dust mitigation - sheesh!) needless to say I went back and blew out the whole box thoroughly.
That was two years ago - I try to keep my computer clean - but they somehow find ways to get filthy. The thing is - those 970s are like the Energizer Bunny...
I once serviced a computer for a heavy smoker. The computer had an unfiltered front intake with a pretty powerful intake fan. It sat next to them on their desk for years. They would blow the smoke from their cigarettes into the front of the case to get it away from them.
The case interior, board, and GPU were covered in thick black and yellow tar. The CPU fan wouldn’t even spin. It was so disgusting that I made them throw it out.