Loud cracking sound at one lower floor baseboard radiator

We live in a 33 year old home with perimeter baseboards, they are the standard one pass copper pipe with aluminum fins all with kick plate and adjustable vent on top. There are 4 zones, 2 up and 2 down. We bought the place about 6 years ago and replaced the original cast iron boiler and separate DHW tank with condensing boiler (on outdoor air reset) and an indirect tank. It has worked well and was really quiet although it doesn’t really condense much due to the higher temperature requirements of the distribution system.

About one year ago the DHW circulator packed it in, so I picked up a new circulator and replaced the faulty one. It has been a few years since I was on the tools but this went ok and everything was back working ok a day later. Shortly after we started getting these extremely loud banging sounds almost like the crack of a whip coming from one of the down stairs radiators. At first I thought it was maybe heat related expansion creating and releasing pressure so I cut a tiny bit of the new engineered flooring that seemed to be tight up against the copper riser pipe to the radiator, this didn’t help. I have opened the bleeder a few times and just straight water comes out not even a hint of air. I tried turning the pump from medium to low speed but no real change.

The cracking sound is definitely coming from the radiator in the front entrance way on the ground floor or in very close proximity to it. The sound started right after the circulator was replaced. This sound seems to increase with boiler temperature, so louder when colder out. There are usually 4 or 5 loud cracks in a period of 3 -4 minutes starting just after the zone valve opens or when the pipe starts getting hot.

The house has two floors all above ground, the ground floor is concrete with tile in most areas and floating engineered floor in our entrance way where the noisy radiator is. The upstairs floor is a concrete skim coat and tile with engineered floor over it. We live in Vancouver so the noise is a problem for about half the year during heating season. I put up with this for a part of last winter and hoped it would just go away but it didn’t. It has now persisted for just over a year and is still so loud it wakes me up at night.

I am not sure what to do or where to look? The boiler is only accessed from the suite downstairs which is rented out so it is hard to get in there and really investigate as much as I would like. The offending radiator is in our space. If it was in the tenants space they would have complained or moved out by now.

Any ideas?