Knocking noises from crank area.
I purchased a trek mountain bike off ebay and the bike is in as new condition, i keep getting knocking noises from the crankset area of the bike when pedalling. I have checked the crankset for tightness and all is ok, i have also checked the seatpost for tightness and pedals and greased both of them, also checked the front and rear quick release on the wheels and these are fine. I only get these noises when pedalling and they come and go and are not there all the time. I would be most grateful for any help or advice