Engine noise on the radio only when OD is disengaged

[MrGrey]

hello, I have an odd problem which I was hoping some of you guys may have some insight into the possible causes.

my na mx5 has an aftermarket headunit (an automedia am3100bt running 2x 6.5" JVC's in the doors and 4x 4" JVC's in the headrests)

the problem i have is the whiney engine noise commonly found with a bad earth but it appears only when i have overdrive disengaged (when I am commuting around town)

next week I will have a sparky installing a boot release and wiring up an underseat sub and would love to be able to point out possible causes of this whine (even though he will most likely install a suppressor)

thanks in advance for any light you may shed on the matter

[na6*dori]

have you tried a better earth?

[SLYDIT]

have you tried a better earth?

+1 there is an earth just in front of the diff on the PPF frame and the earths under the wiper motor. get all of them removed and cleaned along with the one on the engine block.