I've got 2 CD-4k EL inverters that have nice little ldrs built into them so they only turn on when its dark(took me long enough to figure that out, you shoulda seen me wondering why it would only work when it was turned upside down ) But when they're on i hear a high-pitch whine outa them. Is this normal?
"Common" is probably a better word, though both my cheapo inverters (EL string and CCFL) are quiet enough. http://forums.bit-tech.net/showthread.php?t=6519 has one solution, I'd try finding where the noise comes from (probably the transformer, touch things with a rubber-tipped pencil) and applying a bit of silicone rubber sealant or hot-melt nylon between component & pcb to damp vibrations. Don't overdo it, heat may need to escape easily.