Lithium batteries are strange beasts, they're very finicky with high and low temp's. The Chinese lithium's like Shorai, have a few issues with the DC to DC converters which prevent them from charging fully when in use. They recommend using a tender all the time as a consequence.
With that said, I did hear a distinct backfire knock, which is very reminiscent of the beginning failure of the sprague clutch. It appears, these 1198's have some issues with the idler gear pin and sprague clutches, which led them to make a change in the design. There are many threads about this problem on this forum, I'd just look around and you'll see the data.
However I'm still on the battery/charging circuit as being the problem. The characteristics of lithium batteries and how they like to be charged, really throws a monkey in the wrench when diagnosing this issue. If I were you, I'd beg/borrow an OEM battery, just to rule it out. If it has the same problem, then its most likely mechanical. If the issue goes away, then its clear the battery is the problem.