Peugeot 106 1.5 diesel noises?

Apart from the usual chugging diesel noise, the car makes two loud noises. At low revs, it gives a gruff deep sound. This reduces drastically if much higher revs are applied. On motorways, I have to press the accelerator right to the floor to avoid a horrible deep sound, instead I get a less horrible higher pitched sound. On motorways or regular roads, when I take my foot off the accelerator (or release the pressure on it) it takes a long time for the noise to drop in pitch. I would like to identify the problem and fix it or at least find ways to deaden the sound a bit... I can't hear my stereo!