Fan runs constantly

Fans run all the time but, at default speed, not fast enough to be heard except in a very quiet room. Fans are thermostatically controlled; as internal temperatures rise, sensors signal the fans to turn a little faster. Normally this only happens when running processor-intensive tasks like video editing, rendering, and compelling, or running graphics-intensive games.

If the fans are loud from a cold start, try an SMC reset:

How to reset the SMC of your Mac - Apple Support

Be sure to use the links in the article that help you positively identify your sub-model so you follow the appropriate instruction set.

If that does not correct the fan issue and you bought the computer new, it is under warranty. Make an appointment for Apple to evaluate.

Thanks for the follow-up. This may be a clue:

...although mail comes in at 70+ in the CPU usage compared to under 10 for everything else.

Are you running any anti-virus or so-called "cleaning" apps? Mils behavior suggests you are. These apps are not needed and, worse, interfere with the protection you paid Apple to build into the operating system. This interference can cause performance issues, high CPU usage, and may contribute to higher operating temperatures.

If you are not sure what is installed, a good place to start looking for culprits is with a snapshot of your software configuration. Fortunately there is a safe, secure way to do that. 

We can quickly and within the confines of these forums help you determine if software issues are the problem if you use EtreCheck, available free from the Mac App Store here:

https://itunes.apple.com/us/app/etrecheck/id1423715984?mt=12

We can also see hard data about drive performance and RAM usage. Etrecheck is the development of a long-serving and trusted contributor here expressly for displaying information in these forums to help us help you. It will not reveal any personal or secure information.

Run it, select “Report" from the left-hand pane (scroll down to the bottom of that pane to find):

When its report displays, click the "Share Report" icon from EtreCheck’s toolbar and then "Copy report” from the resulting dropdown.

⚠️ Please DO NOT highlight the text in the report before using Etrecheck’s “Copy report” command—that will garble the formatting and make the report slower and harder to evaluate.

NOTE: Dec 2018 changes to the forum software require you use the “Additional Text" icon (see example below) to embed the report into a post:

Paste the report into the resulting “Additional Text” window:

Also see this excellent user tip on posting text reports like EtreCheck. It is better illustrated than mine.

https://discussions.apple.com/docs/DOC-250000211