ford cargo manifold change ?

I have just change my manifold on my 813 cargo, the new one is brand new and so is the gaskets, for some reason the lorry is making a really loud blowing noise asif there is a hole in it somewhere but have checked the manifold and the exhaust down pipe no air at all coming out fort it might of been oil from where it has been sitting for 5 months and it isn't that, the problem we got the cab is fixed down because it is a Horsebox and have limited room to do anything, just wondered if anyone else has had a problem like this before ????