I am going to assume you're talking about an analog feed.
I am going to take a guess what you have isn't related to the LNB assembly or the dish at all, but rather some sort of bad TI. But before I even get to that point, have you tried doing a complete swap on the cables? What I mean is this...
Excuse my text drawing here...
DISH -- CABLE -- RX
H) ----<cable 1>---- (H
V) ----<cable 2>---- (V
Swap cable 1 for cable 2, so H is going through the old V cable and V is going through the old H cable? If the noise changes to H, you got a cable issue. (Breach in the shield near some electric lines, etc)
If it is TI, then there are other things to start looking at. Smaller scale, are there any light poles near the dish? (parking lot lights), or is it aiming towards a traffic light on a utility pole. If you have a light pole near the dish, make sure the light pole's ground is still grounded well. Is it aiming near a motor somewhere, like a AC unit, etc?
On a larger scale, if none of that shows up, are you near an airport or military installation?
One thing to do also is to look on a spectrum analyzer at the feed and see what the noise looks like, you can tell a lot from that sometimes too.
"To the optimist, the glass is half full. To the pessimist, the glass is half empty. To the engineer, the glass is twice as big as it needs to be."