I can feel vibrations/rumbling sometimes

There are times during a burn that sitting in my comfy recliner, which is situated almost directly above the WG in our basement, that I can feel a rumbling in the floorboards and thru the chair itself. Last night it even made the lamp on an end table next to me shake a bit.

What's going on? If you look at the boiler or pipes it does not appear to be vibrating. Too much air into the burn box...or maybe, not enough? This was the first burn of the year so the boiler started out in clean condition.

A few answers to the questions and then my observation today.
Yes, it's on concrete(entire basement floor is concrete), basement is not well sealed. As a matter of fact, I had the interior doors of the basement that lead to the exterior doors above grade open and a window was cracked open too. "Old" part of the house is not sealed as well as the newer part, my chair that I referred to is in the old part. I burn only cord wood and nothing unusual about the loads I've had in there this weekend. I don't think it's the pipes. I can feel the rumble slightly if I put my hand on top of the boiler. The thing about the "rumble" is that you can feel it but can't really hear it until you enter the basement and even then it's very faint.

So, my observation?? I went to the boiler when I felt the "rumble" today and played with the fresh air intake damper. The rumble seems to subside and then go away if I close the damper all the way...almost the opposite of what I thought would happen.

The rumble description is meant to refer to the vibration I can feel...it's not really a rumbling "sound". The only audible effect is when I go down to the basement and then it's very slight and almost seems like the air pressure in the basement is fluctuating.

AHS had told me that if my heat load is not large then running completely closed is fine. Our heat loss is way under what the E100 will put out so I'm going closed and we'll see as this seems to have remedied the situation.

Regarding storage, the shoulder season is about the only time I wish I had storage. Fortunatley I did get the WG with the 4 hour cycle timer and this helps a lot but I'm sure I'm burning more wood than necessary and storage is the remedy to that. It's something that's "on the list" but it's down the road a bit.

On another note, I took the fresh air inlet disc out this summer to give it a good cleaning as it didn't seem to swivel much and also to clean out the tube into the boiler a bit better. After only 2 heating seasons the metal part of the disc facing into the boiler has deteriorated substantially. AHS sells the rubber/silicone part on it's own for about $9 each and also sells the complete disc assembly for......$52! I thought I mis heard him, but no...that's the price. I bought one and I hope it's the last one as I have a friend that is going to see about fabricating some for me. He thinks they have the exact same rubber type material where he works and can cut me the metal parts in SS so all I'll have to do is put them together.

AHS gets parts to me reasonably quick but they are expensive!