Hi, firstly to answer your question - your suspicion that your CX801 in March was operated by B-LXG is correct, quite the coincidence actually! However, I have my suspicions that this issue is not only limited to B-LXG but to the other older A35Ks in CX's fleet.

When I flew on B-LXB and B-LXF in seats 45A and 47A respectively, I noticed a sound that kept on repeating for the first few hours of flight. It was neither high pitched nor low pitched, and it didn't really sound like a fan/motor - I was never really able to put my finger on what the sound was. Just the way you described it, the sound would start and continue for a while, then it would briefly stop, before starting up again. The sound definitely was not present for 90% of the flight in my case... maybe 50% but I am really not sure because I was asleep for a good while mid-flight. From my footage, the sound was definitely there in the first stages of the flight but not the latter stages of the flight. For the record, when I flew on B-LXO (again, in seat 47A), one of the newest A35Ks in the fleet, this sound was not present during any stage of the flight. If we could find a way to get that audio file through, we might be able to figure out if this is the same sound and if it affects only the older A35Ks, or if this is an LXG-specific problem.