As a 17 year old student I was flying a C150 with an instructor. The aircraft had just come from maintenance and it was one I’d not flown before. I told my instructor I could hear a very high pitched noise and could smell engine fumes. He said he couldn’t sense either but would get a second opinion from another instructor.
I stood and listened as he taxied out with another instructor for a circuit. I could definitely hear the noise, it was louder outside. They came back in shortly afterwards and said neither could hear anything unusual but when I told them I’d heard it louder they agreed to get an engineer involved as a precaution.
Turned out that a cylinder head had cracked in three places, between both valve seats and the spark plug. The triangular piece was fretting loose and the engineer reckoned it might suddenly fail if the aircraft was flown again.
That was 47 years ago but it taught me to be very suspicious about anything out of the ordinary.