2004 Sentra transmission problem / hissing sound

[CaliSentra2004]

Well, I am asking for help with the repair of my Sentra. I desperately need help because I cannot afford a major repair at the mechanics. Especially if they would misdiagnose it and I would have to replace more than I need.

Many many thanks!

Short version:
Basically, I had a misfire in one cylinder, but I had to drive it. I replaced sparkplugs, that helped for ten minutes and the problem reoccured. I kept driving. Then second cylinder started misfiring and that took out the car for good. It sat for two weeks and I replaced all four coils. Shazam! The engine is running like new! I can drive it around and there are no shakes whatsoever, revs well. But... I cannot do more than 20 mph. I think transmission does not shifts and I hear the hissing sound from the engine compartment when I drive. What I think is that at the last run when it was going on two cylinders the transmission shut down into the limp mode or got broken (hope not). The other hypothesis I have is that there is some vacuum leak (hence hissing sound), but I do not know how it would make transmission to go into limp mode.

Story version (maybe useful for someone):

I bought my 2004 Nissan Sentra in 2014. It was running well for about half a year. Then just before a major relocation trip the engine started shaking. At first it didn't happen always and I could stop the engine and re-start to bring it back to normal. Went to the shop, but at first there was no OBD codes, so the guys did not diagnose it. After 1 - 2 visits the OBD codes showed up and diagnosis was multiple cylinder misfire. Fast forward a month, the car was at the mechanics for 19 days. They finally replaced sparkplugs which fixed the problem. To treat them fairly, they did more work for me on brakes, but they could not diagnose the shaking problem for long. But they fixed it, so I drove from the East Coast to West. 5 days, reaching 80 - 90 mph (trying to stay close to the limits, though). The ran was flawless. Then, the third day after the reaching my destination, the shaking problem came back. Though this time it was slight and I could drive with it, it was noticeable though. Well, the problem is that I got into financial troubles then and couldn't afford another costly repairs. So I kept driving, wife was insisting on using it, convenience, etc. I tried replacing sparkplugs again, it was a tell all - one sparkplug was darker, oily, the fuel was not burning in that cylinder. My neighbor advised me to change coils too, but it took me some time to get them (I was also very busy). Finally, at one trip my wife calls me that the car could not go. I go there and it is barely moving, no power, shaking like crazy. We got it home finally (short trip, but some pushing was necessary). Again helpful neighbor helped, we were disconnecting coils to see which are not working. 2 out of 4 did not. Then we swapped the coils around and that was it! The coils were deciding which cylinder is firing and which is not. Ok, so I got them. Neighbor helped to replace them. (Great guy!) The engine is running again! Also, it is running pretty well, no noises, smooth RPM transitions. But, I cannot go more than 20 mph. It seems that transmission is locked and does not shift. Also there is a hissing sound coming from engine compartment when I drive. It only starts after I start driving (if I idle in place after starting engine from cold, there is no hissing sound, when I drive it appears and even if I stop it is still going). The only thing I tried is to disconnect the battery for about 20 minutes, thinking that it will reset transmission computer, but I didn't.

[SoundEfx]

Check for vacuum leaks.