I would surely pull the plug and check if you still have an intake valve in there. Might be bent if the spring started to collapse a bit, but that would be a hell of a unique failure on one.
I've heard of cam chain tensioners breaking in them too, much more common to come out of spec and the pop you hear is the intake valve not closing all the way before TDC allowing the fuel in the intake tract to detonate with a pretty good amount of noise.
You can literally bend those little XR valves by hand once they get tweaked a little, so once they are done, they are done. Might want to pull the valve cover and then the head, the two gaskets for the job are less than $20 together. A whole gasket kit for the engine can be had for pretty cheap too. I've gotta rebuild my XR75 some time, no clue why I keep putting it off, it's really easy.