Strange Sound Under the Hood when Foot on Gas
 

I hope someone might have experienced something similar before and would suggest me what is causing this.

Mine is a 1999 Altima and has 122K miles on it. Very recently I noticed some sound under the hood when I put my foot on the gas i.e while accelarating. When I take my foot off the gas pedal the sound goes away. No warning lights on the dash.

It's hard to explain the sound but it's something similar to a motor spinning. It started happening recently.

Any ideas why this is happening?