This sounds like the drain pump, can you run the washer on a rinse cycle and see if you hear the whirring?
Do you have any other symptoms eg noisy when it spins, or the clothes not being cleaned very well etc?
I can only assume its a bearing issue as the speed of the drum changes, its causing friction and thus causing the noise.
If the nose doesnt appear when you complete a rinse cycle this would prove the fact all components are working, and I wouldnt look to do any proactive maintenance unless the noise becomes successive.
I havent heard back from you in a while, do you still require assistance?
I suggest your arrange an engineer out, the fault isnt caused by electrical input if you cant locate the noise with it running with the back off, and you cant replicate it manually rotating the drum, the next step is to get someone out who can induce the fault this way, which is where the skills and experience of a repair engineer come in.
If you would like me to recommend an engineer in your area please provide your postcode.