On another “learn Turkish” website, I have noticed that an ‘r’ when it is at the end of a sentence or before a pause, sounds like an alveolar fricative.  ‘L’ also. This is from all three of the female native speakers reading scripts for the audio. I have a friend who says it is wrong.  He’s not a native speaker but spent eight years in Turkey and has some linguistics training.  It is obvious to me that neither the script readers nor the script writers there know very much about phonetics.  (They claim that any letter has the same sound ALWAYS.)  One of the readers is from Istanbul, one from Bursa.  The other, unknown.

Is this sound a dialect difference? If so, what part of Türkiye is it from?  Must be from some place my friend didn’t go to.

Native speakers of any language tend to think any letter has the same sound always. That is because they do not need to recognize the differences. But lack of phonetic knowledge does not indicate that they would pronounce the sounds wrong.

I think voiceless alveolar fricative belongs to standard Turkish in the positions you described. I don´t know about dialectal differences, though. Someone else will help you I am sure.

I think I understand what you mean. I tried pronuncing words that end in r and l and I noticed that if I continue to blow air after pronuncing the word, I hear a sound like that.

If you keep your tongue in the position of the sound "r" and just blow air without pronuncing the letter, you will hear that sound. 

So I think the reason you hear this sound at the end of the words is because people stop vibrating their vocal cords (because there is no vowel after "r" or "l") but continue to blow air for a while. So you hear that sound. It sounds like voiceless alveolar fricative but they are not the same.