Power Line noise is primarily an AM signal.........
I am almost willing to be that it's a noisy power supply......... and I fear it's a neighbors gadget...or a computer....plasma TV.... it's going to be a NIGHTMARE to get these types to even understand, let alone turn off one their treasured stupid gadgets. Here is a list of some of the stuff.....
Plasma TVs are NASTY noise sources.... avoid them....
I once had a HP scanner that when on made a nasty birdie on 224.98mhz
I have seen some SUPER NASTY birdies from cheap ethernet cables.. this is an easy fix. Just give them a free shielded Ethernet cable.
Plastic computer cases with gimmicky windows. Plastic does not stop RFI. Metal computer cases are great at keeping RFI in the computer.
It could be anything......... I once saw a electric toothbrush make some nasty RFI.
My XYL complains sometimes.....but RFI making gadgets are -not- allowed.... in the house... anything we buy gets a good going over with an HT before purchase.
You could try get that 5/8 wave up higher and get the horizontal lobe away from the noise source.
I can't go much higher. He is my posting I had a couple days ago:
There is a picture of my setup.