This may or may not be the issue but...
The reason I asked what year and model your car was is because that problem sounded very familiar to the one I had with mine. When it got cold, I could hear a slight ticking at idle and the sound you are describing while driving at 3K on the highway. If I remember correctly, it wasn't exactly at 3k RPM but more like 2700 RPM. Does this sound correct? The 96,97 and 98 Civic LXs had the cat. converter integrated with the exhaust manifold. They were very prone to cracking. When they did crack, they made the sound you are describing. The bad news, the part is about $400-$500, plus if it's cracked around the O2 sensor like mine was, you have to replace the sensor which is about another $160, plus labor and you end up at about $700-$800. If your car has less than 80K miles on it, it may still be covered by Honda. They extended the warranty on the manifold because of this problem. Mine is extended to 150K and I've had it replaced twice now.

I hope it's not the case.

It's not really serious, but it may trip a check engine light.