I have discovered some unusual sound from my BMW E39 with M54 engine. I have noticed it when applying gas slightly in second, third and forth gears. I think I do not hear it on fifth gear or first or whenever it is in neutral or whenever gas is not applied.

Could you please listen to this Unusualy loud sound sound I recorded while I lightly shifted from first all the way to forth gear. And please let me know what could be the source of it.

I have changed transmission and differential fluids last weekend, but it did not help at all. Could that be the engine noise and I just do not hear it while it is not in gear?