irrating Freightliner noise
 

got a question
my buddy ,mark, has a 1998 Freight liner.
He has been hearing a squealing from in the dash when he hits the brakes.He is not losing air pressure,just hears a noise.
He says he has replaced 2 sensors in the dash and a valve on the firewall and the noise continues.
Brakes work fine but the noise gets on his nerves.
Any one here know what may be causing this?

tks

Tommy