How have you connected the 300 ohm dipole to the (assume) 75 ohm coax?
May I also assume you used a balun to properly make this transition?
Is your area served by more than one FM broadcast station? If so, are
the other stations also affected by the noise?

If so, you have a locally generated noise problem. Do this: Find a
small, portable, battery powered AM radio (like you'd take to the
ball-park to hear the play-by-play (!). Tune it to a very weak AM
radio station. Note: To do this, you may have to be some distance
*away* from you home, since we've already discovered it has RF noise
polution. As you approach you home, listen for the noise to increase,
while turning the receiver - that's turning with an 'r' - you're using
the receiver as a direction finder - listening for a peak in the noise.


In this way, you likely be able to pinpoint the noise source fairly
quickly.

Likely candidates include:

Light Dimmers
Fish Tank Heaters
Electric Blankets

MW