I've just updated a project from jQuery 2 to jQuery 3.3. I'm running the JQMIGRATE plugin and I keep seeing the following in Firefox's Console:

1. JQMIGRATE: jQuery.fx.interval is deprecated jquery-migrate-3.0.1.js:76:4
2. console.trace() jquery-migrate-3.0.1.js:78:5
   migrateWarn http://[redacted]/jquery/jquery-migrate-3.0.1.js:78:5 get http:// [redacted]
   /jquery/jquery-migrate-3.0.1.js:89:4 at http://[redacted]/jquery/jquery-3.3.1.min.js:2:56916

This is happening when I use effects such as fadeIn() or slideToggle() but wierdly it doesn't happen every time.There definitely nowhere in my code that I'm using jQuery.fx.interval so it must be coming from jQuery itself. Why is jQuery using a deprecated method?