Found a weird oddity today.

I took off the oil filler cap (on cam cover) and there is a loud exhaust sound coming from in there as well as quite a bit of positive pressure (presumably on the exhaust stroke) so I'm thinking there may be an issue ahead. May also explain the blue, turning to grey, turning to none at all, smoke on starting, 5 mins, 10 mins.

Is this sound normal on all LETs? Has anybody got any audio of their LET (or XE) with this cap off?

Possibly a valve sticking???