My Inspiron 7720 emits a constant low hum on hard surfaces - how to stop this?

Hi All

When placed on a hard surface, my Inspiron 7720 emits a constant low hum. It's really irritating and I wonder if it can be stopped? 

I have it seated on a piece of thick card now, which helps a bit sometimes. The loudness of the hum can vary a lot if the laptop is moved only slightly.

I don't think it's a malfunction, but may be a vibration caused by the fan but I'd really like to get rid of this hum as it's really irritating.

Has anyone else come across this, and found a solution?

Thanks in advance
Richard