ELM TRACON Controller reported an aircraft not at the assigned altitude was in an area of higher elevation and questioned the pilot about the aircraft.

Narrative

At XA:26Z; Aircraft X checked on my frequency leveled at 7;000 feet. I in bounded the aircraft to Ithaca Tower and proceeded to handle another aircraft that departed from Elmira airport. Along with the Elmira departure I performed the required point out with BGM approach. Aircraft X requested to descend to 4;000 feet with information 'A' at Ithaca airport. I instructed Aircraft X to descend to 4;000 feet and issued left turn heading 215 vector for visual approach to Runway 32 at Ithaca airport. As I shifted my attention to coordinate a point out with BGM approach; and received control from Syracuse based on Aircraft X's position and where I needed him to fly; I was also in the process of switching the eastbound Elmira departure to ZNY. I quickly looked at Aircraft X and noticed he was in descending at which I noticed the altitude reading 3;400. Immediately I verified with the pilot that he was descending down to 4;000 feet because his altitude read out 3;400 feet. He read back that he was descending to 4;000 feet at which I noticed he started to climb slowly to 3;600 but then stopped his climb.At that point I saw he was in my safe MVA of 3;500 and amended his altitude to 3;500 feet. Then I noticed he started to climb to 3;700 feet so I questioned whether or not he was having a malfunction with his aircraft; or if it was the pilot's ability to fly the aircraft at that point. In my opinion; I felt like the tone of the pilot changed slightly and I just wanted to get him on the ground safely and figure out why he descended lower than 4;000 feet once he was safely on the ground. I did not brasher the pilot as I did not want to add any additional stress to the situation but instead make sure that the pilot landed safely. Keep a tighter scan on my scope. For the pilot I would recommend checking his aircraft equipment prior to any flight and pay closer attention on his instruments.