The best transistor replacement

Here is my setup. The 50Hz comes from the charger. 

Thanks JWx, The tape and heads are OK 

Hello gordonmx

The thunderstorm comes when the recorder is set to REC, but the machine uses JFETs to enter REC mode. No mechanical switches are set. 

All other, mechanical switches are cleaned.

The JFET is a GF405... Not easy to find except from a German shop for  5, 90 Euro each... Have to look for a replacement too 

Right side of C2 goes with the wire somewhere up high - could the noise come from there?

Almost certainly a better choice. In the original design they could have used a BC237 for T6 and T8 but they used a different part. If in doubt it's probably best to respect the intentions of the original designer.

What make and model of recorder is it. Is the complete schematic on the web ?

MK

This is a Grundig TS1000. There are schematics on the web. Not all of them with good quality. The player uses BC550 another place originally too. When the BC550 is better, why not used it here...? T6 T8. 

Goes to the Auto level part. I think they rectify the signals an use this to set the recording level automatically. 

Could be, cause the other channel has made a "bump" (... oh, my English ) onces or twice. But very seldom. Could be something else too. 

Is the issue happening with or without auto leveleing, or in both cases? T6/T8 seems serving the leveling/regular path. I would look at replacing C25/C26, perhaps before doing it just to put a known good cap in parallel to see if it reduces the noise. Are the voltages across R77 and R78 equal?

maybe you can post some sound sample from this issue, then maybe someone with experience can instantly recognize it - maybe some kind of self-oscillation?

Also, you could temporarily bypass the autolevel to see what happens, by lifting the two capacitors C2 and C6, and bridge across with a single capacitor, if you suspect auto-level.

If you don't already have a 'scope (not sure the Phonic product you have there will replace it unfortunately) then it would be worth picking up a low-priced one, since there are some reasonable options at $200 and under.