Great idea. I did get someone to enclose the mic with their hand, but it was still noisy. Unplugging it is dead easy. I'll give it a go today.

I tried to unplug the mike in the cab - no change to the speaker noise in the rops. Did the same with the mic in the rops - also no change. Grounding is the next test. Fingers crossed

 

Ops Shortbus, the course was 20 years ago Super Duty F- series

 

@debe thank you for the reply. I'm not sure they do it the same way today though. Maybe Ford does but Cummins uses the fuel pressure not a separate oil pressure like the Ford manual says. They use the fuel rail pressure and vent it back to the tank. But after more looking it seems the high voltage is common for all electronic injectors now days. Something I didn't know but will keep in mind now. Thanks for posting the Ford info.
And it looks like they are AC powered injectors not the DC I was expecting.

 

Hey guys, I connected the intercom via the filter directly to the battery and the whirring sound during revving dissappeared. I "think" it was not the positive that went via the cigarette lighter, but the negative that was being picked up from the truck frame. Hence ground was the issued. Thanks again everyone. Problem solved.

 

ALL of the electrical noise problems that I have had in vehicles have been alternator whine. The charging current flowing through the resistance of the wires, including any weak connections, creates a voltage drop that results in the noise voltage being added to the DC level.
This means that there are a lot of ways that the whine can enter the sound system.
One option is to not have any of the system use the vehicle ground at all. That means that the entire comm system would have what would be ground connections us instead a wire isolated from the physical ground, EXCEPT for connecting to tha actual battery ground AT THE BATTERY. And also have the power feed taken from a connection AT THE BATTERY. No, this is not a simple thing, but it can work.
Of course, it may also be that there is an inadequate connection between the alternator negative and the battery negative, or between the battery negative and the vehicle frame.

And on one vehicle I found that the whine voltage was much greater because of an open alternator diode.

 

A radio communications forman for a major commercial air cargo company told me to check on the type of wire. (better wire)
He said "we used lots of shielded cable." Hope this was'nt already covered. The alternator filter is installed very close to the noise source (the alternator)

 

Hey guys, I connected the intercom via the filter directly to the battery and the whirring sound during revving dissappeared. I "think" it was not the positive that went via the cigarette lighter, but the negative that was being picked up from the truck frame. Hence ground was the issued. Thanks again everyone. Problem solved.

Glad it is solved. Like a number of folks have pointed out over the years, "Ground" is a mythical zero-resistance zero-impedance continuum that is not like the real world.

 

Hence ground was the issued.

Rule of thumb for audio: Ground *always* is the issue.

ak