Just wondering if its normal for the boost gauge needle to be really wobbly (fluctuating very very rapidly between -17 to -20psi) on idle when its pulling vacuum?

Does everyones do this or should it sit solid?
I'm thinking maybe there is a small air leak somewhere, but i've got a bailey breather venting to atmosphere with the metering head blocked off so I spose it can only be inlet manifold side of things...maybe?

Cheers!