2007 Nissan Sentra A "beating" sound and loud cla

I purchased this vehicle used in June. When I took the car in for its oil change, the guy said a rubber mount for the motor had a tear in it and needed replacing, but it was not critical. I asked why it was tearing (when I took it in the mileage was about 63,000) when the mileage wasn't all that high. Until now, it's been wonderful and made no noise except when turning corners first thing in the morning (like the motor needed to warm up or something) and the mechanic said it was a nice, clean car, in great condition. It has never made a noise which went away after warming up, like this one. And the weird thing is that while it's idling, it doesn't make the noise, only when you give it gas. After you drive it a while, the noise diminishes.

Thanks. I was hoping you'd confirm that. The heading by the way was a beating sound and a loud clack. I've got an appointment with Nissan on Friday. Trouble is they say it's $400 to replace one of them! Thanks, though.