I have an 06 YZ450 that likes to build pressure in the radiators while sitting in the garage. The bike has not been started in a week. This morning I went out to the garage and I heard a hissing sound coming from my bike. As I looked around to see where it was coming from I noticed it was the hose that goes from the water pump to the left radiator making the sound while squirting coolant out like it was over heating. I scratched my head and was saying "What the hell"! I then loosened the radiator cap and a large amount of pressure was released. I then proceeded to check all my hoses and made sure they were tight. I just went out side to grab a beer and the damn thing was doing it again. I loosened the radiator cap again and there was twice the pressure than before. My bike has never overheated while riding until last week so I replaced the coolant. I have replaced the coolant several times before and never any thing like this has happened. Any ideas what it might be?