Loud hum from under the hood-2001 Honda Civic

When driving for sustained periods at 70+ mph, a loud sporatic hum comes from under the hood-passenger side. The local Honda dealer found nothing wrong. I’m planning a 12 hour trip next week through areas with few towns and scarce cell phone signals. Should I be as nervous as I’m feeling?

How many miles on this car? Has all required maintenance been done, in accordance with the schedule in the owner’s manual?

It has about 82000 miles. Yes, all maintenance has always been done and on time. It’s hardly ever had any problems.

Maybe an overloaded alternator? I could hear loud humming from my Ranger’s alternator after towing a tent trailer several hundred miles with lights on. Humming went away after battery brought up to charge. Had no subsequent problems - considered it normal operation. Do you have any electrical accessories causing an abnormally high current draw?

Nothing unusual. I turned off the radio and a/c when it first started so I could listen and try to figure out what was going on. But nothing else. I don’t even have a CD player.

Another possible source: loose exhaust shielding. With age of car there may be some rusted out fasteners and the shielding (just light sheet metal) may be vibrating harmonically. Can you duplicate noise with car in neutral and engine at same RPM? Might be useful to check with hood up and friend behind wheel.

What makes this really odd is that it’s sporatic. It doesn’t always happen when I drive over 70mph, just sometimes and then it comes and goes. It’s almost like something has to heat up to start it-and driving for several hours at sustained speeds does it. I’ll rev it up and see what happens. Thanks.