Bicycle Mechanics - Triple front middle chainring rubbing inner derailer

My bike has a triple front. When pedalling in the middle chainring on the front there is a scraping sound. The chain is rubbing on the derailer on the side closest to the bike (inner side). Interestingly it rubs the most when the rear is in the middle few gears and not at either particular extreme? At slow pedalling it barely rubs but when the speed comes up and it seems to start rubbing more (if that's possible?).

The high (big) and low (small) chainrings are fine in all but the most extreme gear (no rubbing, no resistance, easy, smooth quick changing).

I've tried fiddling with the shifting limit screws (high/low) as per the Barnett manual and managed the make the low/high shifting a bit smoother but found it made almost no difference to the middle chain ring.

I've checked the chain length etc, all looks good to me. The bike itself is only a few months old...

Is there something else I should be adjusting or checking?

Thanks for your advice. If it's nothing easy I think I'll drop it into the LBS and try and learn a thing or two :)

The hi/low screws only affect the limit of travel. The position of the mech is set by the cable tension. It is normal for cables to stretch over the first few weeks and the bike shop should be able to re-adjust it.
Some gear levers have the capacity to trim the position of the mech, others have a simple 3-position indexed system. On some (most) systems there is a threaded barrel adjuster for the cable tension, otherwise, you will have to reset the tension at the mech.

I agree with MichaelW, especially since you mentioned the bike is relatively new. There should be a barrel adjuster somewhere on the front derailleur cable. Look for it either as an "in-line" adjuster, or at a point where the cable meets the frame. There is usually one there.

Shift your bike into the Middle and then the extreme inside (largest cog in the back) and adjust the barrel adjuster to the point where it barely touches the inside of the front derailleur (FD). Then shift the RD to the smallest cog and see if the chain rubs the outside of the FD. Adjust the cable tension of the FD to a compromise between these two. It's a fine line between too much and not enough, and with many 9-speeds, there is usually a tiny amount of rubbing at either end of the spectrum.

Once you middle chainring is adjusted (centered in this fashion) check the small and big rings to make sure they shift well (They Should!). If the chain doesn't want to drop to the small chainring, the inside limit screw may be in too far. Likewise if you can't shift onto the big ring.

Good Luck.
L8R

Thanks very much for your advice. Will give it a go at home tonight...!