It's been a while since I have used silent bidding (used it the 4 months prior to BC in MC). When I use it now, it's posting "High Bid" to Raid chat. Even with the setting on "Silent Bid".
Be careful. "Silent Auction" is like a Silent auction in real life. That is, the high bid is announced without revealing WHO the high bidder is. If you just want to take bids without having the amount being relayed to the raid, use "Silent Non-Auction."