My 2007 Nissan maxima makes a very odd hissing/ grinding noise when it gets above 2000 rpm and stops when it gets above 2500 rpm. The noise seems to be coming from the motor and I can only hear the noise when I have my windows down. I took it to the Nissan dealership and they said it was my timing chain going bad. I thought it was a little sketchy though because they wanted to charge me $1500 to do the job. I got a second opinion from a local mechanic who said the timing chain looked fine. Any ideas what the cause of the noise could be?