How to eliminate the "clicks" of the field worker?

Question

Electronics

I switch the load of 7.5 amperes at 12 volts, with an n-channel mosfet. Gray nichrome, wound on the pipe with a spiral. The opening and closing of the mosfet occurs once a second, and at these moments some kind of click is heard. Subjectively, I could not determine with my ear where and what exactly clicks, where you do not listen everywhere this sound of the same intensity. What can be done to eliminate it? And do you need to do something?

Otherwise, there are no problems. Holds a load and doesn't get hot. The resistor from the gate to the source is 18k, I did not put a resistor to the gate itself, since the gate is connected through an optocoupler (on it somewhere 100-200 ohms).

Answer 1

UPD: The answer has been completely changed.

... 5 pieces of 1mm nichrome paralleled, 5 meters long.
... it took me about 8 turns

From these data it follows that your pipe has a solid diameter, about 200 mm. It looks like this heater of yours is an 8:1 RF transformer, has fairly well flux-coupled windings, and therefore, despite the absence of a magnetic circuit, has a good efficiency. Its main feature is that the secondary winding is short-circuited , i.e. it is a current transformer.
If you connect such a trans to a 12 volt current source through a high-speed switch, then all sorts of interesting effects can be observed in it. The main one is that in the first instant this current will be repeatedly exceeded, and only after the short-circuit current has dissipated in the secondary winding (i.e. in the pipe), the supply current will come to a stationary value of 7.5 amperes.
How can this current peak produce sound? One of the explanations was given in his answer by VT100 - it is not the heater that clicks, but the PSU, due to overcurrent. I give the second answer - a large current gives a strong magnetic field, due to which a magnetostrictive effect appears, deforming the pipe and the nichrome winding (by the way, a strong field at the same time explains the sound when turned off, due to the energy accumulated in the field, but the above process goes in reverse side). You can pull by the ears and the third option - the peak of the current initiates all sorts of resonant effects at frequencies audible to the human ear, but here for verification it is necessary to measure the inductance of the heater, or even better to look with a storage oscilloscope.
How to fix the click? In my opinion, this is elementary: you need to arrange a smooth increase in current in any convenient way. For example, if your field key has a sufficient (i.e. multiple) current and power margin, then open it with a voltage gradually increasing on the gate (say, in 0.1 ... 0.2 seconds, this should be enough), although even with a current / power margin, this will require installing a key on the radiator. You can also saw through a longitudinal groove in the pipe so that the secondary coil is no longer short-circuited.

Answer 2

Nichrome itself is recessed into this sealant tightly.

Then - I bet on the fact that during the transition process the core of the transformer or inductor in the PSU clicks.