My 1965 house has multi-zone baseboard heating (pumped, hot water circulating through baseboard exchangers). This is our first heating season and we have encountered several noise issues.

1. In one of the zones, 2 exchangers (both appear to be cascaded) make noise like a shaking can of marbles -- like gas engine knocking . In case it was water hammering, I bled both exchangers (releasing about a cup of hot water) -- no improvement.
Next, I try to find it there are loose straps, parts rubbing against each other, etc. This was impossible to do as I could not get behind the baseboard shell (thick metal 6” high, not that pressed sheet metal, screws painted over). The side panels are the access areas, but I could only access the ends where the bleed values are. Although I have a basement, I’m told my flooring is concrete and there is absolutely no flexing and I don’t think there are parts rubbing against each other to make a constant audible sound. Using my own hearing (it is not perfect from listening to led zeppelin in my younger days), the sound appears to be internal.
The other exchangers in the same zone are quiet.

2. The other problem I encountered is another zone will make sounds of “running water” -- literally like the vertical riser sound you hear on the 1st floor when you flush a 2nd floor toilet. It wakes up my daughter and she thinks there is water coming in. The only thing I can do is to bleed the system, but it may be the opposite of what it’s trying to do, which is to remain filled with water. I am wondering, just from using my intuition, if this zone is discharging (a lot of) water into a reservoir and then refilling it when the zone is activated by the thermostat.

Also, the exchanger is making a similar noise as the other zone -- a shaking can of marbles.

Does anyone have any ideas – I asked one plumber from the HVAC company, and he basically said there are no fixes (which is his way of saying he doesn’t know what the problem is).