Check the air box cover. On the bottom there is a aprox 3/8" black hose that should be there. When I owned a 400, the dealer did the 1st service work. I had the same problem, me and my son found this hose disconnected. The dealer must have had taken it off and never put it back. I think it goes from the crankcase to the air cleaner. With the hose off it almost sounded like an exaust leak at idle. Thats when I decided to do all the service work myself.