As others have already said, the shutter in the Pentax KX is no louder than the shutter in any of its contemporaries.
The KX was the top of the line 35mm SLR from Pentax at the time ( mid 1970's).
I see that the OP has posted a few similar threads asking "is this or that 35mm SLR form the late 1970's a professional camera".
I think it is probably more important to consider how the camera was used rather than how it was marketed by the mfgr . This is especially true at this distance in time from the original manufacture date. Either camera when used daily by a professional photo journalist is going to be hammered probably to death, by now. However, either on that was owned by an amateur who took care of the gear is probably going to be a more useful tool.
My mom was a journalist for a daily paper. I remember her going through cameras like you and I go through disposable razors...I don't ever remember her using a "professional" camera in her day-to-day work. I remember she had a Pentax MG (far from what anybody would considered professional camera) for a while - perhaps, two years. There were also a many Spotmatics. She seems to have preferred Pentax and Canon cameras with simple user interfaces. She even carried a Canon sure shot for a few years.
Last edited by BradS; 09-17-2012 at 12:25 PM. Click to view previous post history.