Weil-McLain Prepurge Thump (video)

[Biks]

I have a Weil-McLain Ultra Series 3 gas boiler. I'm getting this loud thump when it starts it's PREPURGE cycle. Should it be this loud? Is this normal? Should there be any sound?

 

[Fitter30]

Inducer fan needs to be pulled and inspected along with the heat exchangers. Fan running should be smooth running and just air noise that bang could be delayed ignition.