I had the same issue with 4 LC-equipped locos. My solution was to remove all motor and light control wiring from the LC board, but leaving it installed to control sound only. I installed Train Control Systems M-1 decoders (really small little suckers), set in a "valley" on the LC board, mounted on a piece of double-stick Scotch tape.
Doing so transformed each loco ( 3 Atlas, 1 Kato) to remarkably smooth, quiet runners. For $25 each, the M-1 is a good deal, when you look at $100 for each QSI or Lok Sound (and now Soundtraxx) sound decoders.
The LC sounds are OK, but not in the same ball park as the others.
I bought some of the very early LC boards at around $110 to $140 each. If I were to put them on Ebay, I'd be lucky to get $20. So I "settled" for somewhat lower sound quality for not spending $400 on the 4 locos.
The old Soundtraxx decoders had HORRIBLE motor drives. Just about anything is an improvement. I don't think the Soundtraxx even had silent running, which is probably the hum you heard. Even a fairly basic decoder with silent running is a huge improvement.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
I have an Atlas GP40 and installed a Soundtraxx DSD-AT100LC about a year and a half ago in it when sound decoders were coming into their prime. At first I thought it was fantastic. But after I purchased several locomotives that had the QSI sound chip already installed with speakers...well there's just no comparison. The last straw was when I could hear a "hum" coming from the decoder when the engine would first start out. (Apparently it was always, there, I just never paid attention to it.) I just replaced it yesterday with an NCE DA-SR. Wow, what a difference. Its like a different engine completely. Runs smooth as silk. The sound was annoying anyway even with a baffled speaker I could never get it to sound loud or as good as a QSI.
I still have 2 more engines with soundtraxx decoders in them ,but for now I'll let them go until QSI can bring their pricing down to a more affordable level. A hundred bucks a piece is ridiculous.