I own a 1999 Toyota sienna that has 112,000 miles. For the past few months there has been a whirring noise that seems to be coming from the center engine compartment. It started quietly and now has increased in volume. It reminds me of a speedometer cable that is beginning to fail. I have taken it to my trusted mechanic for a diagnosis and he thought it was metal on metal noise from the transmission. I have owned cars for 40 plus years and have’nt heard of this type of tranny noise. What could be causing this? It makes this noise under power and when I release the accelerator the noise almost disappears.
Also there is a clunking noise coming from the passenger front wheel after releasing the brake pedal when I come to a stop. I have replaced the strut,ball joint,and lower control arm bushing trying to remedy this. I cannot find anything else that is loose.
Also it has an automatic tranny.
That clunking is likely to be driveshaft or brake caliper related. I must assume, since you’re asking about it, that you’re done throwing parts at a problem that hasn’t been diagnosed yet. 
Have you verified the transmission has the a) right fluid, and b) is at the right level? At this mileage, you should have had 3 fluid exchanges so far (one every 30K miles). How many have you done?
It might be your CV joint. Is the rubber boot intact?
Based on the noises, the age, and the mileage, I’d suspect an inner CV joint. The joint is part of the “half shaft” or “axle” assembly. It’s constructed like a three-pronged device with a bearing on each prong, the entire assembly being slid into a three-channeled receiver. It allows axial movement as well as limited articulation under tortional load.
If one is shot, the other is probably on its way. If it is that, it can be verified by the mechanic putting the car in a lift and turning the wheels back and forth. If they thunk abnormally, you’ve found the problem.