An RCD trips when it detects an imbalance in current flow - when more current flows in the live wire than the neutral indicating an escape into the ground circuit.

A combination of damp and dust and spider webs inside an appliance can cause this.

An ageing RFI or radio frequency interference suppressor might cause this.

I have experienced this with an appliance. When connected to an old-fashioned supply without an rcd it worked fine and I never did locate a fault with it. I am tempted to believe some (older) appliances simply shouldn't be used with an rcd. I suggest you ensure the interior is clean and dry and perhaps have a fellow with a portable appliance tester check your heaters for basic safety.