When a hissing sound is heard while pulling a propeller through, what should be the next step?

Hearing a hissing sound while pulling a propeller through typically indicates that there may be an issue with the engine's internal components, such as a potential leak in the cylinder or inadequate sealing due to worn piston rings or valves. Performing a compression check is the appropriate next step because it allows you to assess the health of the engine's cylinders and determine if there is a significant loss of pressure that could lead to poor engine performance. This test will provide valuable information regarding the compression ratios and help identify any underlying issues that might require attention.

Other options such as a visual inspection or replacing the propeller might not directly address the root cause of the hissing sound and would not yield the crucial information needed to assess engine integrity. Adjusting the valve timing could be necessary depending on the specific findings from a compression check, but it is not the immediate action to take without first understanding the condition of the cylinders themselves. Thus, the most logical and effective step is to conduct a compression check to gather the necessary data about the engine's condition.