Why does engine sound louder going faster?

The RPM's of the engine are running higher going down the highway than sitting at an idle. Make sure the exhaust system is in good shape also.

Hmm, are you sure it's just the engine? My question for you is- how did it sound before? 45 mph in high gear, is not that fast for the RPMs to make the engine sound this loud. I agree it might be an exhaust system issue. You should definitely get this checked out. If this is OK, I would look at your differentials. When was the last time you had your car serviced for this?

Differentials? Not at all. You must be thinking of something else, Mark.... If Tera REALLY means ENGINE, then it's rod knock. But that's unlikely unless there's previous oil starvation. COLD-running noise could be piston slap, that disappears after warmup. DRIVELINE speed-related noise is almost always a worn wheel bearing...in any of the 4 locations. If CHASSIS noise, check window seals and roof rack stuff. Exhaust noise from perforated rear Y-pipe (common in these) tends to be loudest when accelerating from a stop, so I doubt it's exhaust. I'll bet on a wheel bearing, as that's the cheapest of the mechanical sources ($200 each corner).

Ernie, your right, I think I was thinking of universal joints. Did you consider that as a possibility?

If not, I agree, wheel bearings, we had them go out on our Toyota Prius and it got noisier as we drove faster. Also, one more thing, we had "cupped tires" on the Toyota and it also made more noise the faster we went. Could be any of these.