Noisy output from opamp comparator
When comparator U4C output goes high (caused by the "A" signal from the first stage rising through the threshold set at "B") there is a reaction that lifts input "C" at U4D a little higher. 

Lifting "C" higher causes the precision rectifiers output level (U4D) to rise and that, in turn causes the "B" signal to rise. That, in turn causes U4C's comparator output to switch back low.

At the start of the sentences above I said U4C output went high but, that caused a chain of events that ended up with it going low pretty much straight away afterwards.

Do you see the problem? The problem is that the actual input ("C") to U4D can be manipulated by the output of U4C and that is because that TACH1 input is fed via R4, a 10 kohm resistor. If that resistor wasn't there i.e. it were a short AND the output impedance of the input voltage at TACH1 were very low then this wouldn't happen.