It's normal. The continuous ping is to allow pfSense to ascertain that your upstream gateway (in this case, it's your modem/ router) o verify that the connection is active and usable.
This is helpful in multi-WAN connections where the router can detect connection failure on one link and switch to the next. It's also used to restart certain services or connections to force downstream services to change their state to reflect the loss of connection.
The ping latency results are also used to generate the link quality RRD graph.
You can change both the frequency and the destination to ping - you might want to change this because your router can be up and contactable but the actual internet link may not be.
To do so, go to System -> Routing -> Gateways. Click the "e" button next to the default gateway.
Under Monitor IP, enter an alternative IP address that is on the internet and contactable through your link. e.g. Your ISP's DNS server IP or Google DNS server IP
Click on Advanced to expand it.
Under Probe interval, enter a new value (in seconds) to change the interval between pings. If you are using an external server, you might want to increase the interval in case this behaviour is deemed to be an attack.