In the R22 I was in cruise and the engine tone changed suddenly. No change to the flight characteristics but the MAP dipped a little. I was 10 minutes from base and came in and landed. Told the engineer what I had experienced and he checked it out, turns out a head gasket developed a small crack and was leaking pressure which explained the change in noise. It would have flown quite a bit more before failing completely but still, it gave ample warning.
enjoyed that, an engineer with a sense of mischevious humour.