Bubbling noise after water pump and belt were replaced

More than likely you have an air pocket in the heater core. Here is a guide that talks about this:

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

Basically, we may need to drain and fill the cooling system again if you are not able to get it purged but I would suggest starting with using a fill funnel as shown below and that should help work the bubbles out.

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

Please see the picture below and then we can take the next step. Let me know if you have questions on this.

Thanks