Heat register basically just piece cover made by plastic or other material. You can just remove it. If there are still noise then it is problematic. The heating pipe is inside wall and floor. It is difficult to reach. There may be a small leak or a piece tape there cause the noise when air is blowing. I don't know a good way to fix it. It is not a problem from heating point of view.
If you removed the register , the noise goes away then the things is simple. You can adjust the register or replace to another style. It should solve the problem. Home depot has the parts.
But there are always some noise from there when air is blowing. I don't know how loud yours is. In night when everything is quiet, I can always hear it. It is normal.
Timothy's- Thank you very much.
Mine is worse than you think. It is from the pipe(it has same level of noise when take heat register out). It may be hard to find a solution and this is why furnace company is not willing to do it. But I will send the complaint to Tarion as it bother me much.