Well one way to find out for sure is to remove the bug shield and get up to 65 or whatever and see if the noise is gone, then go put the bug guard back on and try again. If the noise is back then it is the rubber windshield moulding that needs to be re-sealed. Dealer won't do it as bug guard is aftermarket. If you had the dealer put it on then you might be able to get them to seal the windshield.
Most of us have this problem and it is because the airflow is changed with the bug guard. The rubber moulding is not "tight" at the edges and actually flaps or pops....sounds like you are riding behind a sand truck or big rain drops are hitting your windshield or roof.
Easy fix, however, some have used small double sided tape under the rubber to hold it to the glass better.
Good Luck.