Again, thanks all for the continued tips and ideas on this.
@Changren_Yong I had some Silca drivetrain cleaner so I tried it this weekend. It made a significant difference, and I was hopeful, but ultimately after a ride this morning the noise is still there.
@Michael_LoFaso thanks for lots of ideas. One thing I haven’t mentioned before is that I have a road bike that also has Force E1, and I haven’t ever had this issue. So, I don’t think I’m over-cleaning, as that would probably have caused issues with the other bike. Also, the rear brake is great. I am a loyal worshipper at the Church of Dave Rome, and while it’s possible I was over-enthusiastic with my brake bed-in procedure by virtue of having the motor to assist, I don’t think that’s likely the issue, as the brake was fantastic for a while after initial bed-in. I did buy the bike new, and I’ve been really impressed with Trek’s customer service, but the closest Trek owned store is far enough away that for now my preference is to try and resolve this without going there.
@Carl_Sechrist Is “FOD” Foreign Object Damage? Thanks for the input - that really does seem like a good analysis and explanation of what likely caused this. It would certainly explain why it’s only on one brake, and I’m wondering if maybe that’s why after sanding the pads and rotor I was able to get the noise to stop for a while… until I rode on dirt again and maybe got something else or some dirt embedded in the same area?
My new 180mm rotor and adapter have arrived, and new Galfer pads are due in a few days! Hopefully that will sort it out.
@Michael_LoFaso interesting that you mentioned Sinter pads - I maybe should have ordered those instead of Galfer.
I did, however, decide to pull the trigger on ordering a Sinter brake bedding machine. I have been thinking about this for a few months now and figured now is as good a time as any. This slightly ridiculous purchase (due to the cost) is partly justified by me thinking I can offer brake bed-ins for a few $ and make back part of the purchase cost. So, if anyone reading this is in the West Los Angeles area and wants to make use of it, please reach out to me.
I want to thank everyone again for all the great ideas and input on this issue. I hope none of my response above is taken as push-back or dismissal of good ideas. I appreciate all the input, but I do need to pick and choose which paths I follow amongst all of the options given.