Here's my take on tren cough boys - it only happens when tren enters the blood stream; it hits like a ton of bricks. It is nearly instantaneous and yes, sometimes preceded by a metallic taste. If and when it begins, you will know and you should withdraw your pin so you don't chew yourself up. It doesn't stay with you very long - a couple of minutes, maybe. Wait for the storm to pass and find a new injection site.
If you are mixing your tren, draw the tren first then your test so that there is no residual tren on the outside of your pin. This way, if you do nick a capillary, or worse yet, a vein, there will be no residual tren on the pin to incite the cough. In many cases (and you should always aspirate) you can bypass the nicked capillary or vein and comfortably inject without the cough.
Hope this helps! It could be different for you, but I am pretty sure the cough is standard when this compound enters the blood stream.