Mounting NFS Datastore - Mounting with random port numbers?

I am mounting an NFS datastore to our QNAP. We opened 111/tcp, and 2049/tcp in the firewalls. When trying to mount, the firewall logs show:

Block from $ESXI to $QNAP on port 47110

I can understand the qnap sending back to the source at 47110, but it's sending to the NFS server on that port. On another hypervisor it's sending 42445.

So we've been adding the NFS datastore to all of our hypervisors, watching the firewall logs, waiting for it to get blocked to reveal the randomly generated port number above 40,000 and then adding that port to the firewall, then adding the datastore for the 2nd time.

Anyone else dealing with this? What is this port, and where is it coming from? Our firewall is becoming messy with all these ephemeral ports set as destinations..

Thanks.