Well, for starters, you don't have a timing "belt," but, three timing chains: one primary and two upper, or "secondary," chains. It "could" be a chain rattle, but, without hearing the noise for oneself, it's impossible to say. The timing chains connect the crankshaft to the camshafts, which operate the valve train. The typical timing chain problem on these vehicles results in a "whining" noise. I think a bit more diagnostics needs to take place before discussing what the "fix" is.