My '02 WRX has been making some valve rattling and detonating noises for a little while.. It happens when under 1/2 to full throttle acceleration. I have also noticed that sometimes it will pop back through the intake every now and then. I have changed the plugs, air filter, and fuel filter. On my A/F ratio gauge everything seems fine under throttle and am getting 14 psi boost. I do not have any check engine codes on at all. All is stock except for a turboXS bov. I am assuming that the timing belt may be stretched some. It is definatly time to change it. I have 108k miles on it. Any other ideas??