1986 Chevrolet Blazer loud thumping

When I'm driving, occasionally a loud rhythmic thumping will come from the driver side wheel well, like something is catching with every wheel rotation. When I let off the gas and the vehicle slows a bit it stops, then I can get back up to speed and it wont happen again for a while. Any ideas? Its not a squeak nor does it start and stop with the application of the brakes. It appears to have to do with the speed, though it has happened between 30-60 mph. I would ignore it, but I'm worried it may become more frequent. There is also a lot more vibration of the vehicle than before. It isn't the engine, I had it checked. Thank you for your time.