after taking the car out last night for a bit of a drive (dont get to use it much now i have a van) i parked it up poppd the bonnet just for a quick check over with the engine still running, i noticed the pipe for the nipple on the top of the dv to the inlet was all shriveled like it was sucking hard but not pulling anything through if that makes sence. when i turned it off the pipe went back to normal. when driving it all seems ok, and the dv can be heard on gear change. why is it shriveling up though? dv sticking maybe?

cheers in advance guys.