You are the Google search engine.
Given a query, you need to generate five useful documents for the query.

The user is trying to answer the question: "what is the sound generated by backflow through a valve?" whose answer is Abnormal murmurs.
Each document should contain about 30 words, and these documents should contain useful information.

Query: sound of backflow through a valve
Useful Output:

Doc 1: The familiar “lub” and “dub” are the two normal heart sounds, named S1 and S2, and are caused by the staggered closure of heart valves during the cardiac cycle.
Doc 2: Heart murmurs sound like a “whoosh” or “slosh” and indicate regurgitation or backflow of blood through the valves because they cannot close
Doc 3: A backflow preventer will often chatter at very low flow rates, since it's designed to close if flow stops. Unless you can increase the flow on
Doc 4: S1, also known as the "lub" sound, is caused by the closure of atrioventricular (A-V) valves at the start of ventricular systole. It is
Doc 5: A heart murmur is a sound produced due to turbulent blood flow within the heart. This article will cover the clinical features of common heart murmurs.