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I think its from the drain holes...When driving >20km/hr, a staticy sound is emitted from behind each seat. It quietens when you slow down, and get louder the fast you go, loudest being around ~70km/hr... Last time I cleaned the drain hole, I cut length of electrical cable, fed it through the cabin until I saw it come out the bottom and flossed it fanny style...

Anyway to quiten it? I thought of stuffing cotton down there, but that would eliminate the purpose of a drain hole

I have a similar sound, im thinking that it is just road noise. Just goes to show that we both have squeaky clean holes... (wait for cow to open)

Behind the top seat belt mounts is a drain tube on each side of all NA's NB' and NC's.
Inside the drain tube, near the very bottom, there should be a little flap of rubber. The flap is roughly oval in shape and one end of the oval flap is attached to the inside of the drain tube. Since the flap is oval, it doesn't sit horizontal across the inside of the drain tube, rather, it angles downward. This angle allows water to drain down from the top and also prevents debris and noise from coming up the tube. Many people remove the flap so the tube doesn't become clogged resulting in the noise.
Bit of background reading Here and Here

wow! Excellent =D Miata.net link has great visual representation as well ahaha. Thanks for that de bounce...But what to do in replacing that valve? if it is gone, then, would i need to remove the soft top, pull out the drain and go through the usual sources to get valve again?

edit: if so would it be NA01-R1-786A?

Sorry, no idea how to replace the tubes or how to replace the flaps if they are removed
Might be worth a visit to a MX5 wreckers to look at the best way to get them out.

Your part number looks correct, same as in the NB parts list

Unless someone on the forum wants to chip in

Thanks De bounce motivated me to check my drain holes

Regards
Old Dude