There shouldn't be any hiss when it's "bypassed" all that's in the signal chain at that point is a couple of buffers, they shouldn't add any noticeable noise. What I'd do to try and narrow down what's causing the the hiss is to remove the PT2399, the CD4066 and the LFO opamp.
That leaves just the input and output buffers connected so hopefully the hiss will now be gone. If it is then add the other ICs one at a time, starting with the CD4066, then the PT2399 and finally the LFO opamp, hopefully that will help pinpoint what is causing the hiss.
The electronic switching works like this. The dry signal passes through the first buffer (U3A) and through the 10k resistor to the output buffer/mixer (U3B), this happens whether or not the effect is bypassed. When the effect is "on" U4A and U4D are both closed so the signal can pass through them to and from the PT2399, the delayed signal then gets mixed with the dry signal by U3B. When the effect is "off" in Boss mode U4D is open so no signal can get from the PT2399 to the mixer stage, so you just hear the dry signal. When it's off in tails mode U4A is open so no signal gets to the PT2399, U4D is closed though so whatever signal is currently being processed by the PT2399 is still mixed with the dry signal, to give the tails effect. If you had a lot of noise coming from the PT2399 you would still hear this when bypassed in tails mode. This wouldn't affect Boss mode though and turning the level pot all the way down would stop it.
I wonder if there might be an issue with some CD4066s or more likely with my circuit, because a few people have mentioned problems with the delay signal bleeding through in bypass, mine doesn't have this problem though.