It was 9 p.m., I was trying to read, and I was not alone. There was a sound, like fingernails gently scratching at a smooth surface, coming from the sill of the bay window.

The noise stopped for a moment, and I continued reading. But again, the faint scratching. Perhaps a mouse was setting up housekeeping, in which case I didn't mind sharing quarters.

When I told my husband, however, he was'nt amused. While I evisioned story-book caricatures of soft, furry mice, he saw long-tailed, disease-ridden rodents. We decided to call an exterminator.

Thumbing through the Yellow Pages, my only association with pest control was the TV spot of a handsome, young man in a neat uniform who assured his viewers he could take care of their problems -- if only they would call him right away. I made an appointment for the next day.

Two, not one, mem arrived at the door. Sex appeal aside, they were a marvel in efficiency. One pulled out the garbage and crawled under the sink with his magnifying glass. Moments later, he displayed triumphantly what he said was a mouse dropping. The other inspector climbed our rickety step ladder to look in the attic.

They marched around the exterior of the house, tapping at window frames, and peered into basement corners withh their flashlights. No space was too small or too difficult to reach.

Then came the bad news. With grave faces, they told me I not only had mice, but also carpenter ants and, oh no, termites. Whipping out a glossy brochure with blown-up prints of wood-hungry insects, they casually mentioned they would be in my neighborhood for the next wo days, if I would like to set up an immediate appointment.

It would cost $70 for the first treatment and $22 every month for one year of follow-up treatments (the estimate did not include termite treatment because I had a warranty with another firm). I was not to worry, they added, because they would have so much poison in my house by the end of the year, no insect would dare invade.

I called another firm, recommended by a friend, and they sent an older man dressed in a suit. He, clearly, was not going to crawl under my sink. Together we toured my house. He pointed his flashlight at a black speck and said, "See, there is a carpenter ant." On closer inspection, I saw it was a beetle, but I didn't argue. His receptionist had already diagnosed carpenter ants over the phone, based on my imitation of the "snap-snaps."

We looked at some more dirt balls and black insects (not all moving), and he wrote out a contract. With one year of monthly treatments at $257, he promised to kill carpenter ants, house ants, mice, rats, silverfish and five varieties of roaches.

Next, I called the firm which had issued the termite warranty when we bought our house 11 1/2 months earlier. They arranged to make two separate inspections for termites and for other pests.

After a tour of the house, the first inspector sat back in my arm chair and said in a slow, lazy drawl, "Ma'am, I wouldn't worry about this. If you had a carpenter ant problem, you would see 10 or 20 ants a day. Now, that noise you heard, that might be a mouse. Mice will come in and out of houses in warm weather. It wouldn't do much good to set traps now. Wait and see if you hear any more moise in the fall. Then we can set some traps."

Decidedly, I preferred this inspector's technique. If and when I wanted treatment, it would cost $150 for carpenter ants with a one-year warranty or $45 for mice. I didn't have to take the package deal for all the other pests, nor did I have to contract for 12 monthly treatments. There was no point, he said, in putting "chemicals on top of chemicals which aren't all that safe,"

The second inspector from the same firm also found termites and arranged to treat them with chlordane. The cost was $30 to renew or one-year warranty.

But, the sound in the bay window was not termites and every few days we could still her a "snap-snap, snap-snap sap-snap," Since the inspectors could not agree whether the window residents were mice or carpenter ants, I called a fourth firm.

After directing the inspector to the window and hearing nothing (the situation every time I wanted someone to listen), he raised his hand and said, "Does it sound like this?" He clicked two fingernails together. "Yes, that's it!" I exclaimed. Finally, someone knew what I was talking about.

We examined the outside of the window, but found no sign of ants. "I am sorry to have to tell you this ma'am," said the inspector. "But, I think you have old-house borers."

I had expected the fourth inspection to produce a majority opinion, but now I had a different diagnosis.

The cost? "Well, it's expensive, ma'am. We have to put a tent over the house, and all people, plants and pets have to evacuate for two days (foundation plants can't be saved). We gas the house and it permeates the wood and kills all the borers. It is the only way we can get them. It costs $1,600."

The inspector may have expected shrieks of protest or a look of horror, but it was all I could do to keep from laughing.

I didn't want to call in a fifth firm. We still didn't know wht was under the window sill, and some of the inspectors' scare tactics had left some impression: I couldn't just ignore the noise. What if we really did have old-house borers gnawing away at our investment of thousands of dollars?

The only solution was to rip out the floor of the window sill and look. With hammer and chisel my husband and I pried off moulding and boards, and out swarmed 30 or more large black ants, some carrying white larvae.

We emptied a can of Raid on the fleeing insects and dumped a cup of diazinon powder on the ground under the window. "but we wanted more insurance te ants wouldn't congregate somewhere else in the house. I wasn't convinced we needed to sign the 12 month service contract recommended by the first two firms.

(As for the firm tht had diagnosed old-house borers, I seriously considered reporting them to the Better Business Bureau.)

That left the firm that was going to treat our termites. The advised us to spray the nest ourselves with diazinon. The chlordane, they said, would kill any remaining ants.

Fairfax County Extension Service agent R.N. Waghray also recommended diazinon spray. He said one or two applications are usually sufficient because the queen and the young cannot survive without worker ants.

"It's not clear," he added, "why some firms recommend a 12-month contract for treatment of carpenter ants. That conflicts with our recommendation."

We took the advice, the ants are dead and I can read in peace. Plus, we saved ourselves several hundred dollars, if not much more.

Free recommendations on pest control are available by calling a Cooperative Extension Service office. They operate in every county in the U.S. and the District of columbia. CAPTION: Illustration, no caption