It is normal engine noise coming back out through the intake. If you are hearing it more on the left side that is because the hood vent on the left is connected to the intake. The dual-layer 2008-2009 US intake filter silences the rattle, at cost of air flow. Newer US or Euro filters and aftermarket filters do not.
After 9 months with an aftermarket filter I put the stock 2009 back in.