engine noise on FM radio

I have a 95 blazer LS .
recently It started having some noise in the FM mode but not if the CD player mode .
the tried remedies are :
- checked and disconnested alternator ( no problem there )
- replaced OEM radio with another , same issue .
- checked and disconnected plug wires one at a time . same issue .
- installed inline noise filter on DC input of radio . same issue .
- shielded antenna cable / and /or replaced . did not fix .
noise appears to travel through FM antenna and not DC wiring , a frequency issue .
anyone having any suggestions , I have heard about an ECM issue with grounding ???? or emitting radio frequencies ????:confused:
otherwise car runs perfect , no cluncking or noises anywhere else