unsolicited udp packets

[Tom]

Since switching my router to use google dns server I am getting
unsolicited UDP packets in my logs from the two servers attempting to
open ports 63971, 53318, 55117, ...

Is this normal or is it a sign of some sort of mischief?

[KA9Q]

Are you sure these aren't just responses to queries that you sent to
the Google DNS servers? Some poorly written resolvers will send out
several copies of a query to one or more servers and then stop
listening for responses after the first. The additional responses
bounce off a now-closed firewall, or (even worse) trigger pointless
ICMP port unreachables back to the DNS server.