When I flush my tank air/water comes up through the tube that the float connects to and causes a "whish" sounds. Almost like a studder flush the float drops a little but the toilet dosen't flush just the noice and air in the flush box. Driving me nuts think it may be a venting issue but it seems like if someone has a tap running downstairs it does not do it until the tap stops same for the shower in the room with the toilet if you flush, then shower, it does not make the noise till after you turn off the shower. What should I do first I already covered my vent in the shower with a wet rag and plunged it stopped for a little while but came back.

Thanks in advance for any help :)

It's not a venting problem. The easiest thing to do would be to replace the ballcock. If you wish to work on it yourself shut the water off at the angle stop under the tank.
Remove the three screws and pull the float ball abd stem assembly up. Pry the washer aqnd reseat it firmly, Now place a pan over the open ballcock and turn the water back on to flush out the supply. You should hear a strong stream when you do this. If not click on back and tell me about it. Reassemble and give it a flush. Better now? Good luck, Tom