Something in your post just does not make sense. First of all what is the freq range you are trying to scan for railroad activity? Secondly where do you have the squelch set? And last, what is your definition of a "birdie".
I thought it made perfect sense, but...
First - Freq range is VHF AAR FCC assigned railroad channels (approx. 160-161MHz)
Second - Squelch is set a little past when the radio mutes, pretty much standard
Last - for this note, I meant an interfering non-broadcast carrier, although as pointed out last night, technically a 'birdie' is internally generated.
I will try the antenna removal idea to determine source next time I listen (had a brain cramp and didn't think of that!)
Thanks all...