Wise move.
It may simply be a badly seating valve or ring, but at the end of the day, its been built wrong somewheer down the line and in an ideal world needs to be fixed right away.
A leakdown test performed by a professional who understands what he is doing will pinpont it to valves / rings or headgasket, then you can move forward in an educated position to minimise further expense / risk / heartache.