1999 r reg 1.9 deisel polo

[Landyjay]

Hi I'm a newbie to the forum so would like to say hello

And secondly would like some help with a problem that has been getting worse with my polo
Ok here goes lack of power and black smoke pumping out the exhaust when getting to around 50 mph.
Then I have a noise that has just started from what sounds like the exhaust it's like a hissing sound, if I didn't know better i would think my Tryes where punctured, it only gets louder when my foot is on the gas....
Any ideas !!!!!!!