Knocking noises

Hi there,

I'm normally pretty good at diagnosing and fixing problems on my bike, but this latest one has me really stuck! I'll explain..

It all started on a recent run down to Brighton. When I put the gears into certain combos (middle ring up front, 4th (of 6) at back, for example), I get a knocking sound, perhaps three or four knocks per rotation of the pedals. This varies with the gearing but doesn't seem to correlate to rotation of the cranks or rear wheel.

Checking my bike over when I got home revealed the cones were a bit loose at the back - I tightened them up enough to stop any lateral movement in the wheel but this didn't solve the problem.

Since then it's just got louder and louder. A bit embarrassing on the way to work every day!

Thought maybe it was the bottom bracket. When I take off the chain and run the cranks around however, I can feel a little uneven resistance but they're not grinding or anything.

What's really frustrating is that I can't reproduce the noises at home and, at the moment, can't even tell where they're coming from!

I recently replaced the rear freewheel (shimano tourney type I think) - could that have something to do with it?

Help, before I go completely mad!