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?