I run a Pioneer bluetooth headunit in my 1986 4Runner. Handsfree calls always sound clear on my side, however about half the time the folks on the other end tell me they hear a repetitious hammer-like sound during the calls.

I've replaced the head unit twice now, had 3 different phones and have tried 4 different brands of replacement microphones with no success in eliminating this hammer sound. And sure enough, every time I've tried to call Car Toys, the local shop where I bought it from, they claim that I'm coming through crystal clear. No tap sound whatsoever. And they say they've never heard of such an issue.

I can assure you that the radio is properly grounded. I've also relocated the microphone wire to no longer run under the steering column to no longer be close to the ignition wire.

What I am told is this: the hammer sound does not increase with engine rpm and does not increase or decrease when I play with the heater blower speeds.

This issue does not seem to be documented anywhere on the web. It's driving me crazy. At times I'm told the hammering is so loud that I've had to end calls short. And no, it's not my friends and family playing a cruel joke on me...

I redid all of the chassis grounds when I did the big three mod. The alternator is new, I have OE resistor spark plugs.

Thoughts?