Cleaned sink waste but now it gurgles

[Tomat35]

This first floor sink was very slow at draining so I removed the waste a bend part and sure enough it was clogged full of black gunge.
Now I have put it back after rinsing it out bit the sink gurgles as the last bit of water drains.
It didn’t do this before.
Help please!

 

[oldbuffer]

Looks as if it has an anti-vacuum fitting, top, to the right of where it attaches to the waste. Try taking the top off and cleaning this out as if blocked it may not be allowing enough air in.

 

[Tomat35]

Yes that’s what the extra bit is. I did look at this when I tried to fix the gurgle the first time but I’ll check it more thoroughly. I’ll try the sink without the top on the vent as a test maybe

 

[Tomat35]

Unscrewed the top of the vent, took out the washers cap and insert but there was nothing obstructing the vents.
If I leave the top off it doesn’t gurgle

 

[Elkato531]

A new trap will cost you less than a tenner from a plumbers merchant. Replace it.