74 Swinger brake CLUNK!

My 1974 Swinger has front discs and rear drums. There is a distinguishable “clunk” on the driver side rear drum (or so it seems) as soon as I start to roll forward after putting the car in drive. You can hear it and feel it. It feels like the drum engages and remains that way until I park and shut off the car. I can literally watch the gas gauge move as I drive so something has to be making that drum clamp down. Help please!