Chain (?) noise using middle ring of GXP triple
Last year, my tourer/commuter gained a 48-36-26 Truvativ trekking chainset (and a GXP bottom bracket). I'm at a loss to explain the noise levels, though, when using the middle ring. The first instinct is to trim the front mech (Shimano 105) a little but when you hear it begin to come into contact with the chain (SRAM 9-speed, SRAM 34-11 cassette) you realise that's a different noise.
The noise does seem to "pulse" a little with each pedal downstroke, almost as though a bit of flex under power was causing the chain to rub the front mech, but, as I said, it's definitely not the same noise as occurs when I deliberately adjust the front mech to allow it to begin to rub the chain.
Any ideas what might be causing the problem?
To be honest, though the noise levels are less on the outer ring, and there's no pulsing nature to the sound of the transmission on that ring, I do think the bike is quite a bit noisier than it used to be. It originally came with a square taper Shimano BB and unbranded road triple.