inspiron 1720 loud grating/whirring noise

Can anybody help? Periodically when I have been using my laptop for a while or if it is using a lot of resources I get a loud irregular whirring noise which I think comes from the hard drive. Sometimes the noise is more like a grating sound.

I have run all the diagnostics and the hard drive passes all the tests. In addition the laptop is working as it should do.

I cannot believe it is normal for a computer to make such loud sounds in its normal operation and I am concerned that somwething inside is being damaged. I have owned several computers over the years and this is the first one that has done this.

Anyone got any suggestions. the computer is under warranty.

Regards

Steve