Foul odor coming from insinkerator disposal

Question

We are a Homebuilder and install insinkerator disposals for our new homeowners. I have had 2 complaints from Homeowners that there is a bad odor coming from, their disposals. These complaints come from owners that bought Spec Homes that were about 2 yrs old.We have made several suggestions ranging from lemon peels, bleach, vinegar and baking soda. A Homeowner said today it sorta smelled like rust. How could this be? Please email me if you have suggestions or remedies