Rumbling sound coming from evaporator

This is actually known as bubbling or gurgling noises and it is normally always due to air in the system. If you are hearing this in the dash area, then more than likely it is the heater core.

Here is a guide that will help with this:

https://www.2carpros.com/articles/vehicle-bubbling-noise

What I would suggest is we start with draining and filling the cooling system and make sure there are no pockets.

This guide will give the detail needed to complete this.

https://www.2carpros.com/articles/coolant-flush-and-refill-all-cars

Please run through this info and let us know what you find. However, I suspect this will be an air pocket so please let us know if you have questions. Thanks