The chain tension is highest when accelerating in 1'st gear. If you use the engine to slow down by downshifting does the sound also occur? Check the slack on the chain. If it's loose, which is what I suspect, the problem will go away unless...
You may also have a worn out chain/sprocket set. The only cure is to replace both sprockets plus the chain. DO NOT use any of the old, worn out components with the new components. Doing so will wreck the new parts fast.
When adjusting the chain slack, also check the alignment of the back wheel. If it's off the chain will want to slip off the sprockets.
You may also have a worn out chain/sprocket set. The only cure is to replace both sprockets plus the chain. DO NOT use any of the old, worn out components with the new components. Doing so will wreck the new parts fast.
When adjusting the chain slack, also check the alignment of the back wheel. If it's off the chain will want to slip off the sprockets.