I have an older furnance with a Honeywell RA 89a 1074 switching transformer that has a loud hum at times at other times a low hum When I hear the loud hum the system still works and if I turn off the power switch and turn it back on , the hum changes most the time. It will usually get quieter. Sometimes tapping the box will change it. The loud hum is more frequent but not consistant. It is hard to tell if the transformer or relay is humming Does this loud hum mean the unit should be replaced? Are there any servicable parts or do you replace the entire unit. Thanks.