If the photos were not noisy prior to merging them, then you need to make adjustments to the software prior to merging.
If they were noisy prior to merging or if you don't have the options (or don't know how) to prevent noise, then use a noise-reduction software such as the one built into Lightroom, or buy a stand-alone app like the one from OnOne.
Noise is not equally noticeable in all areas of most images. For example, blue sky shows significantly more noise than say, foliage. Therefore you should not apply an equal amount of noise to the entire image. Instead of making global changes/adjustments, what you'd want to do is make a selection of the area and then apply the appropriate amount of NR. Why? Because the way NR works is by smearing the pixels which can greatly reduce sharpness. Making a selection allows you to crank up the NR in areas that need it while at the same time not adversely affecting other areas of the image where noise is hard or harder to detect.