Is it OK to get a 5400 rpm hdd instead of 7200 for silence.

Solution
Depends on what you are using it for. 5400 RPM hard drives are significantly slower. Also hard drives are not the loud part of the system.
The 5400 RPM drives are much slower in read and write operations creating a slower system over all. The OS will boot slower, react slower, launch programs slower, programs will run slower, this causes a major slow down across every aspect of the system.

Is it possible the 5400 RPM drives are quieter? I have seen some be but it it is not recommended to swat to a 5400 rpm drive for this reason. IF you are looking for quieter then you may want to look into an SSD that are dead silent but have a major speed boost.

Loud drives are more of a thing of the past as they have become much quieter over the past few years. Unlike the older drives that you could hear in the next room, the newer ones are usually only audible if you sitting right next to it while it is in a long read or write operation. Many fans operation will cover up the noise level, not all though.