You have stumbled across a spurious mixing product of (LO*2)-IF. It's the audio sub-carrier from your local (or perhaps not so local) Channel 11 Broadcast Television station. The LO frequency used with all two to ten metre transverters is 116MHz, and the Ch. 11 audio sub- carrier is on 2Ø3.75MHz. Twice the LO is 232MHz, subtract the IF frequency of 28.25 gives a product of 2Ø3.75MHz.
So, what to do? The fix is really quite simple. Matt, KBØVUK, had sent his unit in for service to eliminate the problem. Upon its return, he was quite astounded to find that the fix was a simple series LC trap. I knocked up a replica of the mod in about five minutes and found that whereas I had suffered from the interference at S9+15dB, after installing the trap the interfering signal was totally eliminated.
Note, the following fix will probably work with any 144 to 28MHz transverter that suffers from this specific interference product.
You will need to wind a small inductor. 4.5 turns of 24AWG enameled copper wire on a 3/16" (Ø.1875") diameter form (I used a drill bit). You will also need a decent low-loss piston trimmer capacitor. I used a Ø.5 to 8pF ceramic piston trimmer that I picked up at the Central States VHF conference.
The LC trap needs to be installed ahead of the first RF amplifier, but after the aerial changeover relay. In the case of the new-style DEMI transverter, it is connected between the junction pad of the ACO relay and C68 and the ground plane. Solder one end of the inductor to the ACO relay pad/C68. The other end of the inductor is soldered to the piston trimmer. The other end of the trimmer is then soldered directly to a convenient point at RF ground potential.
Any modification that you perform (including this one) are made at your own risk. If you damage your equipment you cannot blame anyone other than yourself.