This intake silencer used by espar (should work with webasto) is a bit larger, and would provide more noise reduction I think?
http://www.heatso.com/eberspacher-combustion-air-silencer/
On the interior side you can make a heating air muffler out of home store plumbing parts. Basically you need 2 reducing unions. Something like 3-4" size. Take a piece of 4"? pipe and line it with open celled foam about 1/2" thick, some screen to hold it in place might be needed. Put the reducers on both ends and attach to your ducting. The interior diameter of the muffler should be no smaller than the ducts you are using.
If you really want to quiet the pump clicking down you need to ditch the factory mount. Instead wrap the pump in high density foam at least 1/2" thick. Neoprene would work well. Then use a zip ties or a hose clamp to secure it to a bracket or the van body (make sure the angle is correct). The espar mount is completely made of rubber and does a bit better job I think.
To reduce noise transmission through the lines, add a full loop on the inlet and outlet side.