Could possibly be the brake linings intermittently rubbing against the brake disc. This can be caused by 1) a loose brake disc (check that it is securely fastened to the inner wheel hub), 2) loose brake caliper bolts (tighten), and/or 3) linings that are worn or wearing unevenly (check for the reason for uneven wearing, fix, and replace linings). In some cases, the disc could be excessively worn by use or distorted by heat.
With regard to "tightening the nut" be sure to follow the applicable service instructions for the Matco wheelset. The main wheel bearings require a somewhat special technique to tighten the axle nut. http://www.matcomfg.com/TechnicalMan...ns-tp2-23.html
David Heal - Windsor, CA (near Santa Rosa)
EAA #23982 (circa 1965) - EAA Technical Counselor and Flight Advisor; CFI - A&I
RV-12 E-LSA #120496 (SV w/ AP and ADS-B 2020) - N124DH flying since March 2014 - 890+ hours (as of Mar 2020)!
through June 2020.