Sound from withing my Lenovo K5 phone. What is it? Should I take it back to the store?

Take it back to the store immediately before too much time has passed for them to help you… If you keep it for too long before taking it back to them, they may assume you are liable due to dropping ot or something since you kept it and used for so long.

Phones should not make funny noises when you buy it of the store… Don't even try to fix it because the problem can be more imminent and even if some quick fix does stop the noise, the root of the problem can still be present leading to your phone being critically damaged beyond reasonably priced repair which will force you to pay the $200 or whatever through insurance or buy a new phone all together (if no insurance)…