Yes Clockpit, good post.
The approach incidents I had were a bit earlier than 1,000 feet, more like 4 or 5,000. Engineers think it might have been to supple,entary bleed air opening and supplying the packs with air from a different part of the compressor section. This could also be the reason why so many events have been recorded at top of descent when the taps are closed. However, due to the intermittent nature of the problem all they can do is theorise. What needs to be done is replacement of the labirynth seals or of the entire engine(s). Expensive, but if it's gotta be done, it's gotta be done.