Oil tank pressure

75 Mk 3...when I open the oil tank cap while the bike is running there seems to be quite a bit of pressure being released. There is a loud sucking noise and the idle slows down. Seems odd since the oil tank breather goes directly to the original air cleaner box and is not blocked. I cant understand why removing the oil tank cap would release so much pressure from a tank that should already be breathing. Ideas?

Remove the breather hose from the oil tank and see how much pressure there is from that source.