Strange knocking noise

[chris rooney]

Hi. As mentioned in the title, In the past 6 months or so I have developed a strange rhythmic knocking noise what seems to be coming form my bottom bracket area. The strange thing is, this noise only rears its ugly face when i have changed the chain on the middle sprocket and on the 3rd and 4th cog on the rear cassette. in 3rd, i get knock, knock, knock, etc. in 4th i get knock-er-ty, knock-er-ty, knock-er-ty, etc then it disappears when shifting into 5th.

I had a full new drive train fitted just over 18 months ago and i have even put on a new rear wheel. The only thing i can think of is flat bearings but if this was the case, wouldn't the knocking be constant regardless of which gear im in??

Thanks for reading and any suggestions welcome.

 

[MontyVeda]

does it happen only when riding or when the bikes upside down too?

 

[chris rooney]

No. only happens when im on it riding

 

[Paulus]

Is the noise once every pedal revolution, or random? You say it happens when you change gear to the middle, is that from the large chainring? It could be that the front mech or cable needs a lttle adjustment, it could be moving over ever so slightly too far that the chain is just catching the inside part of the mech, and as you change down the cassette the chain bends a little more with each sprocket until the 5th when the noise goes. Check the clearances between the chain and mech with each gear change.

 

[chris rooney]

Tried that. The only way to describe it is its like a bus engine on tick over. I think im starting to feel it through the pedals but in not 100% if it is the cranks or is coming through the frame