[Oldguysrule]

Sounds like the voice coil on the speaker cone is rubbing on the core. Most likely over driven if its present on both speakers. Inspect the cones for possible tears. If you have a digital multi meter measure impedance between + and ground. You should get 4, 6 or 8 ohms. If you don't get anything, speaker is shot. Does it do it with just one source or multiple sources??