crwdns2933897:0crwdne2933897:0

Why does the fan randomly rev up then slows again?

My fan works great but lately it has begun to randomly rev to highest speed for a few seconds then return to the set speed. What's up?

crwdns2934089:0crwdne2934089:0 crwdns2934093:0crwdne2934093:0

crwdns2934109:0crwdne2934109:0

crwdns2853045:0crwdne2853045:0 crwdns2853046:0crwdne2853046:0
crwdns2889612:0crwdne2889612:0 1
crwdns2934285:0crwdne2934285:0