Need to pick a plumber's brain about strange thumping

[louvfx]

Hi!
So for many years our Burnham steam boiler let's us know the heat is on its way up with a song called "Hammer time"
The hammering only lasts for 30 seconds at the beginning of heating up and then it stops. 90% of the noise is coming from the main pipe (about 10 feet away from the boiler.

But this is not why I am posting.
So for the first time we are now noticing a VERY faint "thump" sound in the walls on the 3rd floor directly below the radiator. If I go directly below that radiator to the 2nd floor (inside closet)
I hear it too. However directly below that (in the basement) there is NO noise at all. The "thump" is mainly on 2nd & 3rd floor in the walls.

Observation:
This seems to happen AFTER the boiler has been off for a while. (the radiator is cool or warm to the touch)
The Thumping at its peak repeats every 9 seconds and slowly dissipates to 1 Thump every 1minute until it stops.
By that time, the heat kicks in again and the cycle begins.

At first, I thought something was hitting the outside house, but it seems to be in the VERTICAL pipe in the walls


What the hell is it?
What is causing it??
Any plumbers out there......does this sound familiar?

Any help is appreciated,
Thanks!
Lou

 

[GReynolds929]

Thermal expansion and contraction

 

[louvfx]

That was my guess too. But I always thought that Thermal expansion and contraction was caused by a horizontal pipe not being pitched correctly.
This appears to be a vertical pipe . In theory, shouldn't any water or condensation fall down to the basement?

Plus, the boiler is always off for a long time. Am I wrong to believe that if the radiators are kind of cool, it should be nice and quiet inside the pipes?
Thanks for the feedback