If the clunking noise is happening while the washer is agitating the most likely cause of this problem will be the transmission.
There is a brake that can slip when the washer is agitating but usually that makes a squeal or squeak sound.
The agitator can be making noises but usually that would be described as a clicking sound.
One thing I'd recommend to check this problem is removing the front panel and see if the source of the noise becomes more apparent. The image below contains instruction on how to raise the top and remove the front paenl.
It may be one of the suspension spring has come loose but if the washer is agitating and there is a clunking noise from the transmission then the transmission will have to be replaced.
I hope this information helps to resolve this problem but if you have other questions after making some checks then submit another question with any additional details for further assistance with this problem.