Sucking noise when choke is on

I noticed the other day that when my bike is idling it sounds normal, however when I put the choke on there is a sort of sucking noise coming from under tank. Had a quick look the other day and the airbox is seated properly.

Apart from having a look at the all the connections are on properly is there anything else I should look for?

I get the occasional backfire out the exhaust when I close the throttle or change gear/pull the clutch in. But I've had this long before I noticed this noise.

Thanks