A bit of a burble-hiss is normal.
The noise is the engine working and air passing the various valves (throttle valve all the time, exhaust valve when it starts to open)... you can hear it 'cause you aren't hearing the combustion noises. Or it could be a bit of an insipient backfire.
Ever hear a diesel truck when the driver engages the "jake's brake"? Jake's Brake is a brand name (AFAIK) for a valve that closes off the exhaust pipes to increase engine braking... BRABABABABABABAPBAPBAPBAP... THAT is farting!
