Interference help

Hello. I’m looking for some help. We use vhf handhelds around our business, icom ic-v82 radios to be exact. I have external magnetic mount antennas on the vehicle’s so we can hookup the radios and get more distance if needed. The problem is two of the 3 setups work Great but one has issues when the vehicle is running. When the bus is started I can talk/hear clearly for about 200 yards after that everyone sounds muffled and broken, if I turn the bus off the transmitting/ receiving is great and can talk to someone for several miles as the drive away. I’ve swapped radios and antennas with the other vehicles and get the same problem so I know its something with the bus and not the radio/ antenna. Radio works great when the bus is not started but something is causing interference when the motor is running. What could it be?

I’m assuming the problem is only on receive - you still get the full distance on transmit. Almost certainly locally generated wide band interference generated in the bus. Look for electronics - tachographs, other non-factory fitted equipment which might be oscillating and generating hash. Look for things not in the other vehicles.