The MBTA reports it's figured out a fix for what was causing weird noises to emanate from the wheel area of the new Orange Line trains and so it's, very cautiously, putting one of them back into service today:
Since being taken out of service, we have identified & implemented a fix addressing the cause of the issue. The train will run periodically as we continue to monitor & test the new vehicles.
The refinements & lessons learned now help ensure that this train, and future trains on the Orange & Red Lines, will be able to perform as designed for the duration of their service lives. We appreciate our riders’ patience as we work to return the new vehicles back into service.