It could be dirt buildup on the squirrel cage blades throwing it a little out of balance. Heaters located near the kitchen area where they can suck in airborne grease particles are particularly prone to build up. The fan motor bushings can fail, but that usually produces a high pitched squeal instead of a rumble. Access to the fans and motor differ depending on the brand and model, but both are replaceable if needed.
Dutch
2001 GBM Landau 34' Class A
F53 chassis, Triton V10, TST TPMS
Bigfoot Automatic Leveling System
2011 Toyota RAV4 4WD/Remco pump
ReadyBrute Elite tow bar/Blue Ox baseplate