check side mirrors. When closed you can see there are rubber seals under them. These seals are in the gap between the mirrors and the base, being located on the base. When torn a little in the corners there is awful whistle at speeds arround 60-70 mph. Just tear the seals more or put some felt tape on the base to prevent the torn be like whistle...
check the front side, when you close the mirror, if there is a hole, this is what makes the whistle... front side should have no holes between the seals... this was the problem with mine whistle noise, after cutting little bit more from the rubber seal to mamke the hole bigger - no more whistle...
To be sure this is the issue try closing the mirror and driving with 60-70 mph if there is still whistle - you have different issue. The issue with me wasn't present when the mirrors were closed.