Sudden noise when driving on highway

[Chaz38]

I was driving on the highway, about 70 mph, when there was a loud sound like something rubbing on the body. It stopped after 5 seconds, then happened again a few hunred yards further on.

[sparky34kv]

Did you check around the wheels when you got where you were going? Did you see anything caught under your car or feel any vibration or was it just the sound?

[petrified.rabbit]

the seals on the doors and or mirrors make the same sound as a seal barking somewhere around that speed?

I tend to account it to a speed / pressure change due to wind resistance. Is it coming from that area of the car?

[Chaz38]

There was no rubbing on the wheels and I couldn't see anything loose underneath.
Thinking back on the incident, (and many, many years ago) I was coasting down a hill , foot off the gas when the noise started.
I felt nothing on the steering wheel, no vibration, , , nothing! The second time the noise started I moved the steering wheel left and right a little, still felt nothing. Now then , on the thinking back bit, the old cars used to do this (Model A's etc) under the same conditions! So, I think it was a muffler or like article vibrating. I'm taking it to the garage tomorrow to get the tire pressures checked, and also to see if they can come up with an answer or seeing anthing loose when its up on the hoist.
At almost 80 years old, I'm not into laying on the ground and checking it out myself.
Many thanks for the question, though. I'll let you know if someone comes up with the answer.