Hi Peter ...

manufacturing plant about 850m away and an  NDB about 900m away.

NDB 900m away: once I had a problem - noisefloor rise -when I used to live
4Km away from a 720KHz AM (100KW). All filtered but the PLL noise of such AM
station spread above and bellow its frequency. Ok, -70dBc but bellow 100KW
...

Other noise floor raiser I had: an Elektrisk Bureau receiver, good piece of
equipment (15KHz to 30MHz) but 2 x 1N4007 to protect the pre-amp were place
before the input preselector raise noise floor when conected to a beverage
antenna. The NDBs/MW are from 6 to 25Km from here.

On this case, a low pass filter solve the problem. On the first case ...
only a loop to null it.

73
Marcus
PY3CRX/PY2PLL
S. B. do Campo - GG66rf
http://py.qsl.br
Active from "DC" to 24GHz