Ok here's one for you.

I have a 2000 Altima. I have this weird knocking noise that happens when I get up to speeds over 20 or so. The rate of the knocking increases to about 3-5 knocks per second when I get up to highway speeds and sort of resonates. It'll get loud and more pronounced for about 5 seconds, then fade for about 5 seconds.

You can feel the knocking in the gas peddals and steering wheel.

There's no vibration like a wheel out of balance. Just a knocking.

Applying the brakes doesn't change anything. Turning doesn't do anything. I tried putting it nuetral and reving the engine. Doesn't change anything. I just had alignment checked and replaced front tires. Didn't increase or decrease the knocking.

Any thoughts on what it might be?

 

CV Joints. Most of the time they will act up when turning, but the loose ones will do as you've described.

Although 3 to 5 beats per second would be a little frequent for a CV joint.

Hope this helps,

Bob