I’m not sure if this is the same issue with your machine, but sometimes mine makes a lot of noise when the steam boiler starts up.
You will know that these machines do not have a vacuum breaker. Instead, when the steam boiler is energised, the valve on the steam wand opens very briefly to eliminate the vacuum in the steam boiler. A good idea in many ways as conventional vacuum breakers do get jammed.
You can hear the valve click when this happens.
I think that sometimes for no reason, this does not work The steam boiler then heats with a partial vacuum inside which means it rumbles and makes a lot of noise until it is up to temperature. I assume this is the result of the vacuum inside.
As the Techno uses temperature not pressure to control the boilers the steam boiler still heats correctly.
Switch off, the then switch on again in normal mode. See if that fixes the problem ?
I don’t need a replacement yet, but would be interested to know the source of your replacement heaters.
Hope this helps.