There are several causes of that.. When it happened to me I took the advice of the singer and went "Up On The Roof". Only that's as far as I got in that song.
Took the outer cover off
Took the condenser cover off
Looked in and said "YUCK"
Grabed the soft brush and the air jet (Hooked to the compressor down below) and made like Gabriel and BLEW (Dirt out of the condenser)
When I got done I blue the crud out of everythnig else crudded up.
Restored covers
Went back inside
Turned it back on
Thatr was like April or May (Forget which) it's still running.