Ah that explains it! How is it generating so much noise (which is very noticeable as you mention) without any airflow? Is it more realistic that proper ventilation can be installed, assuming actual cooling is too expensive?
When they were introduced it was described as “forced air cooling” or similar. It doesn’t work at all well and, as you’ve noticed, makes a hell of a racket.
When Connex/SE Trains first introduced the 376s they stated that air conditioning wasn’t viable due to the doors opening and closing on metro ops - clearly that wasn’t strictly true based on the London Overground 378s which came in only four years later. Sadly are no plans to retrofit the 376s based on what we’ve seen of the planned interior refurbs.
It’s certainly nice when a 707 turns up at the moment!