New Noisy Neighbor right above my unit


I have been living with pounding/stomping noises from above, especially living room, for a awhile. Reported to HDB but useless, the HDB staff said will investigate and that's it.
Went to talk to upstairs neighbours and they said its not them, that they also sometimes hear the noise.
One day they came down to my unit and asked why i complain to HDB against them when they already said its not them (so HDB did investigate but no conclusion). We had a fruitful discussion and confirm that the noise is not from them cos when they were with me, the noise happened.
So it is strange, the noise sounds like coming from directly above but it is actually not. Dunno how to pinpoint the source of the noise.
rooftop is it? banlgadash working?