Rythmic Thumping noise from rear speakers

[Cinken9769]

Why is there a rhythmic thumping sound coming from two of my speakers?

 

[Iced-D]

Ground loop interference? Bad cables? Too many gummies?

 

[Ekea]

whats the approximate frequency? once every couple of seconds? several times a second?

 

[seasick]

Do you hear the noise when at rest (Motors off)
or
Only with motors running
and
I f\\f so, does the rate of the thumping change with motor revs?

Have you always had the sound?
If not, have you disconnected and reconnected the speaker leads?

 

[enfish]

Have you installed a new fishfinder recently? I had a FF one time that you could hear the tranducer pulses on the speakers. The head unit was actually putting out EMI over the air that the speaker wires picked up like an antenna. If I wrapped the head unit in aluminum foil, the pulses went away...