The problem began last summer about 2 hours into a ride on rolling hills when I went over a small but sharp bump across road. I have tried to identify and repair problem since then.
I have replaced the head set, had a wheel mechanic check front and rear hubs, but not take apart. I have had a couple of bike mechanics check the bottom bracket. A small bit of roughness can be felt on the right side ceramic BB bearings but mechanics say they should be OK.
I have changed front wheels but noise still happens at speed, usually triggered by a bump; on downhills it usually at speed over 35km/h but has happened at slower speeds once the problem starts on a ride.
I have removed the water bottle holder and the odometer sensor on front left fork and the magnet on wheel spoke but the noise still happens.
This past winter I had what I will call an "MRI" on the bike frame and forks. A problem was identified in the area close to bottom bracket and chain stay where a chain drop damaged the area. This happened in Sept 2019 with a gouge but it's not deep. According to bike mechanics who looked at the damage in 2019 I was advised it should be OK but to cover exposed carbon fibre with paint and or clear coat which I did over 2019-20 winter and I rode the bike in 2020 and the first half of 2021.
I also converted to an 11 speed from a 10 speed and installed a new drive train in 2020. Bike mechanics and a wheel expert said the rear hub on my existing wheel could handle 11 speed cassette. The MRI and repairs were made by the carbon fibre expert this past winter. He warrants his work. I replaced all the cables and housings this spring.
The problem has returned this week on my 4th or 5th ride. The noise starts almost randomly, triggered usually by speed, downhill, and a bump. I have noticed the bolt holding the front brake had become a little loose and wondered if this could be problem as position of brake pads may have moved to one side. Tried again after tightening but noise still happens at speed, usually. It starts whether coasting or pedalling, and stops whether I slow down or not, whether I brake or not, may last longer than 10 seconds or less than a second.
I am pretty sure the noise is in the front of bike but with the way carbon bikes resonate cannot be completely certain. I do not want to replace the bike, not yet, without trying other less expensive solutions first. Any suggestions?