“Sneezing” on startup?

[Roccaas]

About once every five times I start the car, it cranks, and just as the engine catches, there is a sneeze sound from under the engine (front, passenger side).

Almost like a vent or valve opens to let off some pressure.

Nothing untoward happens, and it occurs randomly, no pattern, no way to anticipate, both cold and warm cranking.

Weird! Ideas? Thanks!